Fix a crasher that could happen if ClangASTSource::CompleteType() found a type...

Fix a crasher that could happen if ClangASTSource::CompleteType() found a type whose name matched, but came from a different language. We need to verify that any types we find are clang types before trying to extra a clang::QualType and then use it.

<rdar://problem/24138711>

llvm-svn: 265443