fix several problems with asm renaming, by pulling it into the mangling code:

fix several problems with asm renaming, by pulling it into the mangling code:

1. it wasn't applying to definitions, only declarations, e.g. int x __asm("foo")
2. multiple definitions were conflicting, they weren't getting merged.
3. the code was duplicated in several places.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67442 91177308-0d34-0410-b5e6-96231b3b80d8