Revert "Refactor compile-time assertion checks in c.h"

Revert "Refactor compile-time assertion checks in c.h"

This reverts commit b7f64c6, which broke the fallback implementation for
C++.  We have discussed a couple of alternatives to reduce the number of
implementations for those asserts, but nothing allowing to reduce the
number of implementations down to three instead of four, so there is no
benefit in keeping this patch.

Thanks to Tom Lane for the discussion.

Discussion: https://postgr.es/m/20200313115033.GA183471@paquier.xyz