Avoid a crash in the test suite where if we have a ObjC "id" type and we ask...

Avoid a crash in the test suite where if we have a ObjC "id" type and we ask it for the PointeeType() we would get a CompilerType with a valid TypeSystem, but invalid opaque type, and ClangASTContext::IsClangType() previously would return true saying that it was a valid clang type and it would crash in removing qualifiers from a clang::QualType that was constructed with an NULL opaque type.

llvm-svn: 247641