Replace pg_restrict by standard restrict

Replace pg_restrict by standard restrict

MSVC in C11 mode supports the standard restrict qualifier, so we don't
need the workaround naming pg_restrict anymore.

Even though restrict is in C99 and should be supported by all
supported compilers, we keep the configure test and the hardcoded
redirection to __restrict, because that will also work in C++ in all
supported compilers.  (restrict is not part of the C++ standard.)

For backward compatibility for extensions, we keep a #define of
pg_restrict around, but our own code doesn't use it anymore.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/0e3d8644-c01d-4374-86ea-9f0a987981f0%40eisentraut.org