Tweak implementation for allowing ObjC builtin type redefinitions.

- Replace string comparisons with pre-defined idents.
- Avoid calling isBuiltinObjCType() to avoid two checks. 
- Remove isBuiltinObjCType(), since it was only used in Sema::MergeTypeDefDecl().
- Have Sema::MergeTypeDefDecl() set the new type.

This is a moidified version of an patch by David Chisnall.

llvm-svn: 55990