Revert "[Clang] Ignore BTFTag attr if used as a type attribute"

This reverts commit b875343.

Per discussion in https://reviews.llvm.org/D111199, instead to make
existing btf_tag attribute as a type-or-decl attribute, we will
make existing btf_tag attribute as a decl only attribute, and
introduce btf_type_tag as a type only attribute. This will make
it easy for cases like typedef where an attribute may be applied
as either a type attribute or a decl attribute.

clang/include/clang/Basic/Attr.td

@@ -1836,7 +1836,7 @@ def BPFPreserveAccessIndex : InheritableAttr,
   let LangOpts = [COnly];
 }
 
-def BTFTag : DeclOrTypeAttr {
+def BTFTag : InheritableAttr {
   let Spellings = [Clang<"btf_tag">];
   let Args = [StringArgument<"BTFTag">];
   let Subjects = SubjectList<[Var, Function, Record, Field], ErrorDiag>;

clang/include/clang/Basic/AttrDocs.td

@@ -2019,10 +2019,6 @@ targets. This attribute may be attached to a struct/union, struct/union field,
 function, function parameter or variable declaration. If -g is specified,
 the ``ARGUMENT`` info will be preserved in IR and be emitted to dwarf.
 For BPF targets, the ``ARGUMENT`` info will be emitted to .BTF ELF section too.
-
-The attribute can also be used as a type qualifier. Right now it is accepted
-and silently ignored in order to permit the linux kernel to build with the
-attribute.
   }];
 }
 

clang/lib/AST/TypePrinter.cpp

@@ -1704,7 +1704,6 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::UPtr:
   case attr::AddressSpace:
   case attr::CmseNSCall:
-  case attr::BTFTag:
     llvm_unreachable("This attribute should have been handled already");
 
   case attr::NSReturnsRetained:

clang/lib/Sema/SemaType.cpp

@@ -8127,12 +8127,6 @@ static void processTypeAttrs(TypeProcessingState &state, QualType &type,
     case ParsedAttr::IgnoredAttribute:
       break;
 
-    case ParsedAttr::AT_BTFTag:
-      // FIXME: Linux kernel may also use this attribute for type casting check,
-      // which clang doesn's support for now. Let us ignore them so linux kernel
-      // build won't break.
-      attr.setUsedAsTypeAttr();
-      break;
     case ParsedAttr::AT_MayAlias:
       // FIXME: This attribute needs to actually be handled, but if we ignore
       // it it breaks large amounts of Linux software.

clang/test/Sema/attr-btf_tag.c

@@ -40,11 +40,3 @@ int __tag2 __tag3 foo(struct t1 *arg, struct t2 *arg2);
 int __tag1 foo(struct t1 *arg __tag1, struct t2 *arg2) {
   return arg->a + arg2->a;
 }
-
-void * convert(long arg) {
-  /* FIXME: the attribute __tag1 is accepted but didn't really do type conversion
-   * or enforce type checking. This is to permit linux kernel build with btf_tag
-   * attribute.
-   */
-  return (void __tag1 *)arg;
-}