Don't crash when mangling types defined in ObjC class extensions.

The original test case here was mangling a type name for TBAA,
but we can provoke this in C++11 easily enough.

rdar://13434937

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179153 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed