Refactor compile-time assertion checks in c.h
This commit refactors and simplifies the definitions of StaticAssertStmt,
StaticAssertExpr and StaticAssertDecl. By unifying the C and C++
fallback implementations, this reduces the number of different
implementations from four to three.
Author: Michael Paquier
Reviewed-by: Georgios Kokolatos, Tom Lane
Discussion: https://postgr.es/m/
20200204081503.GF2287@paquier.xyz