[OpaquePtr] Allow globals with opaque pointer value type

Do this by making opaque pointers a valid pointer element type,
for which we implicitly create an opaque pointer (moving the logic
from getPointerTo into PointerType::get).

We'll never create something like a "pointer to opaque pointer",
but accept it in the API, because a lot of code reasonably assumes
that you can create a pointer to pointer type.

Differential Revision: https://reviews.llvm.org/D104902