Replace __builtin_types_compatible_p with _Generic

Replace __builtin_types_compatible_p with _Generic

_Generic is the C11 standard equivalent (and superset) of
__builtin_types_compatible_p, so by replacing the latter with the
former, we can now rely on this working on all compilers, instead of
previously just with GCC-compatible ones.  And we can drop a configure
test.

This affects StaticAssertVariableIsOfType() and
StaticAssertVariableIsOfTypeMacro() and indirectly unconstify() and
unvolatize().

Neither _Generic nor __builtin_types_compatible_p works in C++, so
this does not change that, but this adds an explicit code comment
about that.

There are some subtle behavior changes, but these do not affect cases
that are in use or likely to be useful.  _Generic does lvalue
conversion on the controlling expression, which means it drops
top-level qualifiers and converts arrays to pointers.
__builtin_types_compatible_p on the other hand, supports arrays and
just ignores qualifiers.  So before, StaticAssertVariableIsOfType(x,
const int) might have worked, but now it does not.  (But note that it
would previously have succeeded even if x was a non-const int, so this
usage would always have been dubious.)  Also,
StaticAssertVariableIsOfType(y, char[]) would have worked, but now you
need to write char *.  (But this is not backward compatible, because
char * would previously not have succeeded.)  Similarly, unconstify of
non-pointers, like unconstify(const int, x) would previously have
worked, but this was just by accident and never useful (you can just
assign directly, without a cast), and the C++ implementation rejects
non-pointers anyway.  Some comments are added to explain this a bit.
There are no current uses affected by this.

Note that even though we have required C11 since f5e0186f865, we have
not made use of _Generic until now (except in an MSVC-specific case in
commit 59c2f03d1ec).  This now raises the effective compiler
requirement on the trailing edge slightly from GCC 4.8 to GCC 4.9.
This in turn means that we effectively drop support for RHEL 7.

Author: Peter Eisentraut <peter@eisentraut.org>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CA+hUKGL7trhWiJ4qxpksBztMMTWDyPnP1QN+Lq341V7QL775DA@mail.gmail.com