MC COFF: Emit the 'b' section flag for .bss sections in GNU assembly

Without this, assembling clang's disassembly would produce an object
file with the IMAGE_SCN_CNT_INITIALIZED_DATA section characteristic
rather than the uninitialized one.  link.exe would warn when merging
comdats with different flags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197529 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCSectionCOFF.cpp

@@ -54,6 +54,8 @@ void MCSectionCOFF::PrintSwitchToSection(const MCAsmInfo &MAI,
   OS << "\t.section\t" << getSectionName() << ",\"";
   if (getKind().isText())
     OS << 'x';
+  else if (getKind().isBSS())
+    OS << 'b';
   if (getKind().isWriteable())
     OS << 'w';
   else

test/MC/COFF/bss_section.ll

@@ -4,3 +4,6 @@
 
 @"\01?thingy@@3Ufoo@@B" = global %struct.foo zeroinitializer, align 4
 ; CHECK: .bss
+
+@thingy_linkonce = linkonce_odr global %struct.foo zeroinitializer, align 4
+; CHECK: .section .bss,"bw",discard,_thingy_linkonce