WinCOFF: Emit common symbols as specified in the COFF spec

Summary:
Local common symbols were properly inserted into the .bss section.
However, putting external common symbols in the .bss section would give
them a strong definition.

Instead, encode them as undefined, external symbols who's symbol value
is equivalent to their size.

Reviewers: Bigcheese, rafael, rnk

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3324

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205811 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCObjectFileInfo.cpp

@@ -548,6 +548,10 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
 
 
 void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
+  // The object file format cannot represent common symbols with explicit
+  // alignments.
+  CommDirectiveSupportsAlignment = false;
+
   // COFF
   BSSSection =
     Ctx->getCOFFSection(".bss",