Using the integrated assembler we'd fail to change section to the
.tbss section for zerofill thread locals. Make sure we do this
before emitting the zerofills.

Fixes PR15972.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187913 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 0c59286..12c3574 100644
--- a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -368,9 +368,10 @@
     MCSymbol *MangSym =
       OutContext.GetOrCreateSymbol(GVSym->getName() + Twine("$tlv$init"));
 
-    if (GVKind.isThreadBSS())
+    if (GVKind.isThreadBSS()) {
+      TheSection = getObjFileLowering().getTLSBSSSection();
       OutStreamer.EmitTBSSSymbol(TheSection, MangSym, Size, 1 << AlignLog);
-    else if (GVKind.isThreadData()) {
+    } else if (GVKind.isThreadData()) {
       OutStreamer.SwitchSection(TheSection);
 
       EmitAlignment(AlignLog, GV);