MC: Support COFF image-relative MCSymbolRefs

Add support for the COFF relocation types IMAGE_REL_I386_DIR32NB and
IMAGE_REL_AMD64_ADDR32NB for 32- and 64-bit respectively. These are
similar to normal 4-byte relocations except that they do not include
the base address of the image.

Image-relative relocations are used for debug information (32-bit) and
SEH unwind tables (64-bit).

A new MCSymbolRef variant called 'VK_COFF_IMGREL32' is introduced to
specify such relocations. For AT&T assembly, this variant can be accessed
using the symbol suffix '@imgrel'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179240 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCExpr.cpp

@@ -250,6 +250,7 @@ StringRef MCSymbolRefExpr::getVariantKindName(VariantKind Kind) {
   case VK_Mips_GOT_LO16: return "GOT_LO16";
   case VK_Mips_CALL_HI16: return "CALL_HI16";
   case VK_Mips_CALL_LO16: return "CALL_LO16";
+  case VK_COFF_IMGREL32: return "IMGREL32";
   }
   llvm_unreachable("Invalid variant kind");
 }
@@ -285,6 +286,8 @@ MCSymbolRefExpr::getVariantKindForName(StringRef Name) {
     .Case("dtpoff", VK_DTPOFF)
     .Case("TLVP", VK_TLVP)
     .Case("tlvp", VK_TLVP)
+    .Case("IMGREL", VK_COFF_IMGREL32)
+    .Case("imgrel", VK_COFF_IMGREL32)
     .Default(VK_Invalid);
 }