[Clang] Directly create opaque pointers

In CGTypes, directly create opaque pointers, without computing the
LLVM element type. This is not as straightforward as I though it
would be, because apparently computing the LLVM type also causes a
number of side effects.

In particular, we no longer produce diagnostics like -Wpacked for
typed (only) behind pointers, because we no longer depend on their
layout.

Differential Revision: https://reviews.llvm.org/D152505