Don't use mangleCXXRTTIName in TBAA for C code.

With r185721, calling mangleCXXRTTIName on C code will cause crashes.
This commit fixes crashes on C testing cases when turning on struct-path TBAA.

For C code, we simply use the Decl name without the context. This can
cause two different structs having the same name, and may cause inaccurate but
conservative alias results.