[llvm-debuginfo-analyzer] Add support for DW_FORM_implicit_const.

The ELF reader ignored attributes with the DW_FORM_implicit_const
form, which caused logical elements to have missing logical data
(mainly line/file declaration numbers).

The issue is described by
  llvm/llvm-project#57040

where GCC generates the following abbreviations:

.debug_abbrev contents:
Abbrev table for offset: 0x00000000
[1] DW_TAG_formal_parameter     DW_CHILDREN_no
        DW_AT_decl_file DW_FORM_implicit_const  1
        DW_AT_decl_line DW_FORM_implicit_const  2

[2] DW_TAG_typedef      DW_CHILDREN_no
        DW_AT_decl_file DW_FORM_implicit_const  1
        DW_AT_decl_line DW_FORM_data1

llvm/lib/DebugInfo/LogicalView/Readers/LVELFReader.cpp

@@ -247,6 +247,13 @@ void LVELFReader::processOneAttribute(const DWARFDie &Die, LVOffset *OffsetPtr,
   const DWARFFormValue &FormValue =
       DWARFFormValue::createFromUnit(AttrSpec.Form, U, OffsetPtr);
 
+  // We are processing .debug_info section, implicit_const attribute
+  // values are not really stored here, but in .debug_abbrev section.
+  auto GetAsUnsignedConstant = [&]() -> int64_t {
+    return AttrSpec.isImplicitConst() ? AttrSpec.getImplicitConstValue()
+                                      : *FormValue.getAsUnsignedConstant();
+  };
+
   auto GetFlag = [](const DWARFFormValue &FormValue) -> bool {
     return FormValue.isFormClass(DWARFFormValue::FC_Flag);
   };
@@ -293,12 +300,12 @@ void LVELFReader::processOneAttribute(const DWARFDie &Die, LVOffset *OffsetPtr,
     CurrentElement->setBitSize(*FormValue.getAsUnsignedConstant());
     break;
   case dwarf::DW_AT_call_file:
-    CurrentElement->setCallFilenameIndex(*FormValue.getAsUnsignedConstant());
+    CurrentElement->setCallFilenameIndex(GetAsUnsignedConstant());
     break;
   case dwarf::DW_AT_call_line:
-    CurrentElement->setCallLineNumber(
-        IncrementFileIndex ? *FormValue.getAsUnsignedConstant() + 1
-                           : *FormValue.getAsUnsignedConstant());
+    CurrentElement->setCallLineNumber(IncrementFileIndex
+                                          ? GetAsUnsignedConstant() + 1
+                                          : GetAsUnsignedConstant());
     break;
   case dwarf::DW_AT_comp_dir:
     CompileUnit->setCompilationDirectory(dwarf::toStringRef(FormValue));
@@ -331,12 +338,12 @@ void LVELFReader::processOneAttribute(const DWARFDie &Die, LVOffset *OffsetPtr,
     CurrentElement->setCount(*FormValue.getAsUnsignedConstant());
     break;
   case dwarf::DW_AT_decl_line:
-    CurrentElement->setLineNumber(*FormValue.getAsUnsignedConstant());
+    CurrentElement->setLineNumber(GetAsUnsignedConstant());
     break;
   case dwarf::DW_AT_decl_file:
-    CurrentElement->setFilenameIndex(
-        IncrementFileIndex ? *FormValue.getAsUnsignedConstant() + 1
-                           : *FormValue.getAsUnsignedConstant());
+    CurrentElement->setFilenameIndex(IncrementFileIndex
+                                         ? GetAsUnsignedConstant() + 1
+                                         : GetAsUnsignedConstant());
     break;
   case dwarf::DW_AT_enum_class:
     if (GetFlag(FormValue))
@@ -573,12 +580,9 @@ LVScope *LVELFReader::processOneDie(const DWARFDie &InputDIE, LVScope *Parent,
       if (abbrCode) {
         if (const DWARFAbbreviationDeclaration *AbbrevDecl =
                 TheDIE.getAbbreviationDeclarationPtr())
-          for (const DWARFAbbreviationDeclaration::AttributeSpec &AttrSpec :
-               AbbrevDecl->attributes())
-            if (AttrSpec.Form != dwarf::DW_FORM_implicit_const)
-              // We are processing .debug_info section, implicit_const
-              // attribute values are not really stored here, but in
-              // .debug_abbrev section. So we just skip such attrs.
+          if (AbbrevDecl)
+            for (const DWARFAbbreviationDeclaration::AttributeSpec &AttrSpec :
+                 AbbrevDecl->attributes())
               processOneAttribute(TheDIE, &CurrentEndOffset, AttrSpec);
       }
     };

llvm/test/tools/llvm-debuginfo-analyzer/DWARF/pr-57040-ignored-DW_FORM_implicit_const.test

@@ -0,0 +1,65 @@
+; Ignored attributes with DW_FORM_implicit_const.
+; https://github.com/llvm/llvm-project/issues/57040
+
+; Output generated by g++ (Debian 11.3.0-3) 11.3.0
+
+; .debug_abbrev contents:
+; [1] DW_TAG_formal_parameter     DW_CHILDREN_no
+;         DW_AT_decl_file DW_FORM_implicit_const  1
+;         DW_AT_decl_line DW_FORM_implicit_const  2
+
+; [2] DW_TAG_typedef      DW_CHILDREN_no
+;         DW_AT_decl_file DW_FORM_implicit_const  1
+;         DW_AT_decl_line DW_FORM_data1
+
+; Attributes with DW_FORM_implicit_const being ignored by the ELFReader,
+; causing {Parameter} and {TypeAlias} to omit line numbers.
+
+; test.cpp
+;  1  using INTPTR = const int *;
+;  2  int foo(INTPTR ParamPtr, unsigned ParamUnsigned, bool ParamBool) {
+;  3    if (ParamBool) {
+;  4      typedef int INTEGER;
+;  5      const INTEGER CONSTANT = 7;
+;  6      return CONSTANT;
+;  7    }
+;  8    return ParamUnsigned;
+;  9  }
+
+; RUN: llvm-debuginfo-analyzer --attribute=level,format,producer \
+; RUN:                         --print=scopes,symbols,types \
+; RUN:                         %p/Inputs/pr-57040-test-dwarf-clang.o 2>&1 | \
+; RUN: FileCheck --strict-whitespace -check-prefix=ONE %s
+
+; ONE:      Logical View:
+; ONE-NEXT: [000]           {File} 'pr-57040-test-dwarf-clang.o' -> elf64-x86-64
+; ONE-EMPTY:
+; ONE-NEXT: [001]             {CompileUnit} 'test.cpp'
+; ONE-NEXT: [002]               {Producer} 'clang version 14.0.6'
+; ONE-NEXT: [002]     1         {TypeAlias} 'INTPTR' -> '* const int'
+; ONE-NEXT: [002]     2         {Function} extern not_inlined 'foo' -> 'int'
+; ONE-NEXT: [003]                 {Block}
+; ONE-NEXT: [004]     5             {Variable} 'CONSTANT' -> 'const INTEGER'
+; ONE-NEXT: [003]     2           {Parameter} 'ParamBool' -> 'bool'
+; ONE-NEXT: [003]     2           {Parameter} 'ParamPtr' -> 'INTPTR'
+; ONE-NEXT: [003]     2           {Parameter} 'ParamUnsigned' -> 'unsigned int'
+; ONE-NEXT: [003]     4           {TypeAlias} 'INTEGER' -> 'int'
+
+; RUN: llvm-debuginfo-analyzer --attribute=level,format,producer \
+; RUN:                         --print=scopes,symbols,types \
+; RUN:                         %p/Inputs/pr-57040-test-dwarf-gcc.o 2>&1 | \
+; RUN: FileCheck --strict-whitespace -check-prefix=TWO %s
+
+; TWO:      Logical View:
+; TWO-NEXT: [000]           {File} 'pr-57040-test-dwarf-gcc.o' -> elf64-x86-64
+; TWO-EMPTY:
+; TWO-NEXT: [001]             {CompileUnit} 'test.cpp'
+; TWO-NEXT: [002]               {Producer} 'GNU C++17 11.3.0 {{.*}}'
+; TWO-NEXT: [002]     1         {TypeAlias} 'INTPTR' -> '* const int'
+; TWO-NEXT: [002]     2         {Function} extern not_inlined 'foo' -> 'int'
+; TWO-NEXT: [003]                 {Block}
+; TWO-NEXT: [004]     4             {TypeAlias} 'INTEGER' -> 'int'
+; TWO-NEXT: [004]     5             {Variable} 'CONSTANT' -> 'const INTEGER'
+; TWO-NEXT: [003]     2           {Parameter} 'ParamBool' -> 'bool'
+; TWO-NEXT: [003]     2           {Parameter} 'ParamPtr' -> 'INTPTR'
+; TWO-NEXT: [003]     2           {Parameter} 'ParamUnsigned' -> 'unsigned int'