[DWARF5][COFF] Fix wrong relocation in .debug_line

Dwarf 5 allows separating filenames from .debug_line into a separate
.debug_line_str section. The strings are referenced relative to the
start of the .debug_line_str section. Previously, on COFF, the
relocation information instead caused offsets to be relocated to the
base address of the COFF-File. This lead  to wrong offsets in linked
COFF files which caused the debugger to be unable to find the correct
source files.

This patch fixes this problem by making the offsets relative to the
start of the .debug_line_str section instead. There should be no
changes for ELF-Files as everything seems to be working there.

llvm/lib/MC/MCDwarf.cpp

@@ -360,7 +360,12 @@ void MCDwarfLineStr::emitRef(MCStreamer *MCOS, StringRef Path) {
   size_t Offset = addString(Path);
   if (UseRelocs) {
     MCContext &Ctx = MCOS->getContext();
-    MCOS->emitValue(makeStartPlusIntExpr(Ctx, *LineStrLabel, Offset), RefSize);
+    if (Ctx.getAsmInfo()->needsDwarfSectionOffsetDirective()) {
+      MCOS->emitCOFFSecRel32(LineStrLabel, Offset);
+    } else {
+      MCOS->emitValue(makeStartPlusIntExpr(Ctx, *LineStrLabel, Offset),
+                      RefSize);
+    }
   } else
     MCOS->emitIntValue(Offset, RefSize);
 }