Add macro to cast away const without allowing changes to underlying type.

Add macro to cast away const without allowing changes to underlying type.

The new unconsitify(underlying_type, var) macro allows to cast
constness away from a variable, but doesn't allow changing the
underlying type.  Enforcement of the latter currently only works for
gcc like compilers.

Please note IT IS NOT SAFE to cast constness away if the variable will ever
be modified (it would be undefined behaviour). Doing so anyway can cause
compiler misoptimizations or runtime crashes (modifying readonly memory).
It is only safe to use when the the variable will not be modified, but API
design or language restrictions prevent you from declaring that
(e.g. because a function returns both const and non-const variables).

This'll be used in an upcoming change, but seems like it's independent
infrastructure.

Author: Andres Freund
Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de