Suppress various new compiler warnings.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 9fb9691a88ae8df9bc30e0f0f72de7c96e73e125
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2021-03-21T15:50:43Z
Releases: 14.0
Suppress various new compiler warnings.

Compilers that don't understand that elog(ERROR) doesn't return
issued warnings here.  In the cases in libpq_pipeline.c, we were
not exactly helping things by failing to mark pg_fatal() as noreturn.

Per buildfarm.

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