Refactor compile-time assertion checks for C/C++

Hi all,

As of [1], I have been playing with the compile time assertions that
we have for expressions, declarations and statements.  And it happens
that it is visibly possible to consolidate the fallback
implementations for C and C++.  Attached is the result of what I am
getting at.  I am adding this patch to next CF.  Thoughts are
welcome.

[1]: https://www.postgresql.org/message-id/201DD0641B056142AC8C6645EC1B5F62014B8E8030@SYD1217

Thanks,
--
Michael