Don't cache the artificial type for the this pointer, there's no

difference in the qual type. This is a workaround for the fact that
the type isn't artificial but the this decl is, however, we don't
have any way of representing it in the current metadata. For now,
however, just don't cache the full type.

Fixes rdar://10831526 and probably a couple of others.

llvm-svn: 150159