Don't use mangleCXXRTTIName in TBAA for C code.

This changes the TBAA code so it doesn't use mangleCXXRTTIName in C,
because it doesn't really make sense there.  Also, as sort of a
defense-in-depth change, fix the mangler so it handles C RecordDecls
correctly.

No tests because I don't know the TBAA code well enough to write a test,
and I don't know how else to trigger mangling a local struct in C.

Fixes a crash with r185450 reported by Joerg Sonnenberger.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185721 91177308-0d34-0410-b5e6-96231b3b80d8