Implement a more sensible strategy for ObjC built-in types (addressing a long standing FIXME in Sema::GetObjcIdType()).

This removes several gross hacks to work around the previous "lazy" behavior.

Two notes:
- MinimalActions still needs to be taught about the built-in types (This breaks one of the -noop test cases). I started this, then added a FIXME.
- I didn't convert Sema::GetObjcProtoType() yet.

llvm-svn: 43567