Refinements to Sema::GetObjcIdType()...

- Cache the typedef, not the type (avoids importing AST/Type.h).
- Emit an error if "id" cannot be found.
- Comment the routine and add a FIXME to reconsider how we emulate GCC's new fangled behavior. This isn't a priority for now, since almost no code depends on having "id" built-in.
- Add a test.

llvm-svn: 42845