[MC] Produce proper section relative relocations for COFF in .debug_frame

[mstorsjo]

To clarify this change (it's rather obscure in this plain form even though the change in principle is trivial); the third parameter to Streamer.EmitSymbolValue is bool IsSectionRelative = false. For ELF, these debug sections are mapped to address zero, so a normal, absolute address relocation works just fine, but COFF needs a section relative relocation, and COFF is the only target where needsDwarfSectionOffsetDirective() returns true.. This matches how EmitSymbolValue is called elsewhere in the same source file.

I'll add something to this effect in the commit message as well.

[labath]

Though I'm not very familiar with this code, I am pretty sure it's correct as the same pattern is used in other dwarf section references too.

llvm/lib/MC/MCDwarf.cpp

if (IsEH) {
     const MCExpr *offset =
         MakeStartMinusEndExpr(Streamer, cieStart, *fdeStart, 0);
     emitAbsValue(Streamer, offset, 4);
   } else if (!asmInfo->doesDwarfUseRelocationsAcrossSections()) {
     const MCExpr *offset =
         MakeStartMinusEndExpr(Streamer, SectionStart, cieStart, 0);
     emitAbsValue(Streamer, offset, 4);
   } else {
-    Streamer.EmitSymbolValue(&cieStart, 4);
+    Streamer.EmitSymbolValue(&cieStart, 4,
+                             asmInfo->needsDwarfSectionOffsetDirective());
   }
 
   // PC Begin
   unsigned PCEncoding =
       IsEH ? MOFI->getFDEEncoding() : (unsigned)dwarf::DW_EH_PE_absptr;
   unsigned PCSize = getSizeForEncoding(Streamer, PCEncoding);
   emitFDESymbol(Streamer, *frame.Begin, PCEncoding, IsEH);
 

llvm/test/MC/COFF/cfi-sections.s

+// RUN: llvm-mc -filetype=obj -triple x86_64-mingw32 %s -o - | llvm-objdump -r - | FileCheck --check-prefix=COFF_X86_64 %s
+// RUN: llvm-mc -filetype=obj -triple i686-mingw32 %s -o - | llvm-objdump -r - | FileCheck --check-prefix=COFF_I686 %s
+
+.cfi_sections .debug_frame
+
+f1:
+        .cfi_startproc
+        nop
+        .cfi_endproc
+
+f2:
+        .cfi_startproc
+        nop
+        .cfi_endproc
+
+// COFF_X86_64: RELOCATION RECORDS FOR [.debug_frame]:
+// COFF_X86_64-NEXT: {{.*}} IMAGE_REL_AMD64_SECREL .debug_frame
+// COFF_X86_64-NEXT: {{.*}} IMAGE_REL_AMD64_ADDR64 .text
+// COFF_X86_64-NEXT: {{.*}} IMAGE_REL_AMD64_SECREL .debug_frame
+// COFF_X86_64-NEXT: {{.*}} IMAGE_REL_AMD64_ADDR64 .text
+
+// COFF_I686: RELOCATION RECORDS FOR [.debug_frame]:
+// COFF_I686-NEXT: {{.*}} IMAGE_REL_I386_SECREL .debug_frame
+// COFF_I686-NEXT: {{.*}} IMAGE_REL_I386_DIR32 .text
+// COFF_I686-NEXT: {{.*}} IMAGE_REL_I386_SECREL .debug_frame
+// COFF_I686-NEXT: {{.*}} IMAGE_REL_I386_DIR32 .text