[llvm-gsymutil] Fix assert failure on FileEntry.Dir empty (#79926)

Summary:
FileEntry.Dir can be empty if debug info only contains relative path.
This caused an assertion failure when gsym segmentation is trying to
copy a file entry with empty dir. As the fitst entry of StringTable is
always empty (and is preserved), `StringOffsetMap` doesn't have key 0.
Hence, `find(0)` returns `End` and `operator->()` fails the assertion

Test Plan:
./bin/llvm-lit -sv llvm/test/tools/llvm-gsymutil/X86/elf-empty-dir.yaml

llvm/lib/DebugInfo/GSYM/GsymCreator.cpp

@@ -55,7 +55,10 @@ uint32_t GsymCreator::copyFile(const GsymCreator &SrcGC, uint32_t FileIdx) {
     return 0;
   const FileEntry SrcFE = SrcGC.Files[FileIdx];
   // Copy the strings for the file and then add the newly converted file entry.
-  uint32_t Dir = StrTab.add(SrcGC.StringOffsetMap.find(SrcFE.Dir)->second);
+  uint32_t Dir =
+      SrcFE.Dir == 0
+          ? 0
+          : StrTab.add(SrcGC.StringOffsetMap.find(SrcFE.Dir)->second);
   uint32_t Base = StrTab.add(SrcGC.StringOffsetMap.find(SrcFE.Base)->second);
   FileEntry DstFE(Dir, Base);
   return insertFileEntry(DstFE);

llvm/test/tools/llvm-gsymutil/X86/elf-empty-dir.yaml

@@ -0,0 +1,108 @@
+## Test converting DWARF using relative path
+
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-gsymutil --convert %t -o %t.gsym --segment-size=100000 --num-threads=80 --quiet 2>&1 | FileCheck %s --check-prefix=CONVERT
+
+# CONVERT: Input file: {{.*\.yaml\.tmp}}
+# CONVERT: Output file (x86_64): {{.*\.yaml\.tmp\.gsym}}
+# CONVERT: Pruned 0 functions, ended with 1 total
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_X86_64
+DWARF:
+  debug_str:
+    - ''
+    - main.cpp
+    - foo
+  debug_abbrev:
+    - ID:              0
+      Table:
+        - Code:            0x1
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_name
+              Form:            DW_FORM_strp
+            - Attribute:       DW_AT_language
+              Form:            DW_FORM_udata
+            - Attribute:       DW_AT_stmt_list
+              Form:            DW_FORM_sec_offset
+        - Code:            0x2
+          Tag:             DW_TAG_subprogram
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_name
+              Form:            DW_FORM_strp
+            - Attribute:       DW_AT_low_pc
+              Form:            DW_FORM_addr
+            - Attribute:       DW_AT_high_pc
+              Form:            DW_FORM_addr
+  debug_info:
+    - Length:          0x27
+      Version:         4
+      AbbrevTableID:   0
+      AbbrOffset:      0x0
+      AddrSize:        8
+      Entries:
+        - AbbrCode:        0x1
+          Values:
+            - Value:           0x1
+            - Value:           0x2
+            - Value:           0x0
+        - AbbrCode:        0x2
+          Values:
+            - Value:           0xA
+            - Value:           0x1000
+            - Value:           0x1050
+        - AbbrCode:        0x0
+  debug_line:
+    - Length:          66
+      Version:         2
+      PrologueLength:  31
+      MinInstLength:   1
+      DefaultIsStmt:   1
+      LineBase:        251
+      LineRange:       14
+      OpcodeBase:      13
+      StandardOpcodeLengths: [ 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1 ]
+      Files:
+        - Name:            main.cpp
+          DirIdx:          0
+          ModTime:         0
+          Length:          0
+      Opcodes:
+        - Opcode:          DW_LNS_extended_op
+          ExtLen:          9
+          SubOpcode:       DW_LNE_set_address
+          Data:            4096
+        - Opcode:          DW_LNS_advance_line
+          SData:           9
+          Data:            0
+        - Opcode:          DW_LNS_copy
+          Data:            0
+        - Opcode:          DW_LNS_advance_pc
+          Data:            16
+        - Opcode:          DW_LNS_advance_line
+          SData:           1
+          Data:            0
+        - Opcode:          DW_LNS_copy
+          Data:            0
+        - Opcode:          DW_LNS_advance_line
+          SData:           1
+          Data:            0
+        - Opcode:          DW_LNS_copy
+          Data:            0
+        - Opcode:          DW_LNS_advance_pc
+          Data:            64
+        - Opcode:          DW_LNS_advance_line
+          SData:           1
+          Data:            0
+        - Opcode:          DW_LNS_extended_op
+          ExtLen:          1
+          SubOpcode:       DW_LNE_end_sequence
+          Data:            0
+...