Enable assembly output of local commons for AIX

Summary:
This patch enable assembly output of local commons for AIX using .lcomm
directives. Adds a EmitXCOFFLocalCommonSymbol to MCStreamer so we can emit the
AIX version of .lcomm assembly directives which include a csect name. Handle the
case of BSS locals in PPCAIXAsmPrinter by using EmitXCOFFLocalCommonSymbol. Adds
a test for generating .lcomm on AIX Targets.

Reviewers: cebowleratibm, hubert.reinterpretcast, Xiangling_L, jasonliu, sfertile

Reviewed By: sfertile

Subscribers: wuzish, nemanjai, hiraditya, kbarton, MaskRay, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64825

llvm-svn: 368306
diff --git a/llvm/lib/MC/MCSectionXCOFF.cpp b/llvm/lib/MC/MCSectionXCOFF.cpp
index 9cee1cc..a3f79c3 100644
--- a/llvm/lib/MC/MCSectionXCOFF.cpp
+++ b/llvm/lib/MC/MCSectionXCOFF.cpp
@@ -28,9 +28,11 @@
     return;
   }
 
-  if (getKind().isCommon()) {
-    if (getMappingClass() != XCOFF::XMC_RW)
-      llvm_unreachable("Unsupported storage-mapping class for common csect");
+  if (getKind().isBSSLocal() || getKind().isCommon()) {
+    if (getMappingClass() != XCOFF::XMC_RW &&
+        getMappingClass() != XCOFF::XMC_BS)
+      llvm_unreachable("Generated a storage-mapping class for a common/bss "
+                       "csect we don't understand how to switch to.");
     if (getCSectType() != XCOFF::XTY_CM)
       llvm_unreachable("wrong csect type for .bss csect");
     // Don't have to print a directive for switching to section for commons.