Bitcode: Don't create comdats when autoupgrading macho bitcode

Don't infer COMDAT groups from older bitcode if the target is macho,
it doesn't have COMDATs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226546 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/weak-macho-3.5.ll

@@ -0,0 +1,11 @@
+; RUN:  llvm-dis < %s.bc| FileCheck %s
+
+; weak-macho-3.5.ll.bc was generated by passing this file to llvm-as-3.5
+; The test checks that LLVM does not place weak GlobalVariables into Comdats for
+; macho object files, they don't support it.
+
+target triple = "x86_64-apple-macosx10.9.0"
+; CHECK: target triple = "x86_64-apple-macosx10.9.0"
+
+@x = weak global i32 0
+; CHECK: @x = weak global i32 0{{$}}