Track LLVM 15 changes.

Track LLVM 15 changes.

Per https://llvm.org/docs/OpaquePointers.html, support for non-opaque
pointers still exists and we can request that on our context.  We have
until LLVM 16 to move to opaque pointers, a much larger change.

Back-patch to 11, where LLVM support arrived.

Author: Thomas Munro <[email protected]>
Author: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/CAMHz58Sf_xncdyqsekoVsNeKcruKootLtVH6cYXVhhUR1oKPCg%40mail.gmail.com