pgsql: Add pg_assume(expr) macro

Add pg_assume(expr) macro

This macro can be used to avoid compiler warnings, particularly when using -O3
and not using assertions, and to get the compiler to generate better code.

A subsequent commit introduces a first user.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3prdb6hkep3duglhsujrn52bkvnlkvhc54fzvph2emrsm4vodl@77yy6j4hkemb
Discussion: https://postgr.es/m/20230316172818.x6375uvheom3ibt2%40awork3.anarazel.de
Discussion: https://postgr.es/m/20240207203138.sknifhlppdtgtxnk%40awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d65eb5b1b84e9104144b6b07b526bc73e819d6d7

Modified Files
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src/include/c.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)