Restructure the API in Type based on a conversation with Richard Smith.

This makes 'isPointerLikeType' a little less confusing, and pulls the
decay check into a separate interface that is much more clear and
concrete. Also, just implement these as logical wrappers around other
predicates. Having a switch based implementation isn't likely to be
necessary. We can try to optimize them later if they show up on
a profile.

llvm-svn: 133405