pgsql: 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 <thomas.munro@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAMHz58Sf_xncdyqsekoVsNeKcruKootLtVH6cYXVhhUR1oKPCg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c2ae01f695b1605bc5e3908ff52b24fce6636caa

Modified Files
--------------
configure                        | 89 ++++++++++++++++++++++++++++++++++++++++
configure.ac                     |  3 ++
src/backend/jit/llvm/llvmjit.c   | 18 ++++++++
src/backend/jit/llvm/meson.build |  3 ++
4 files changed, 113 insertions(+)