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