c: Add _Maxof and _Minof operators

c: Add _Maxof and _Minof operators

Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3628.txt>

These operators have not yet been accepted by the C Committee, so this
is implemented as a GNU extension.

gcc/ChangeLog:

	* doc/extend.texi (Syntax Extensions): Document _Maxof & _Minof.

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_reswords): Add _Maxof & _Minof keywords.
	(c_maxof_type, c_minof_type): New functions.
	* c-common.def (MAXOF_EXPR, MINOF_EXPR): New trees.
	* c-common.h (enum rid): Add RID_MAXOF & RID_MINOF constants.
	(c_maxof_type, c_minof_type): New prototypes.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_maxof_or_minof_expression): New func.
	(c_parser_unary_expression): Add RID_MAXOF & RID_MINOF cases.
	* c-tree.h (c_expr_maxof_type): New prototype.
	(c_expr_minof_type): New prototype.
	* c-typeck.cc (c_expr_maxof_type): New function.
	(c_expr_minof_type): New function.

gcc/testsuite/ChangeLog:

	* gcc.dg/maxof-bitint.c: New test.
	* gcc.dg/maxof-bitint575.c: New test.
	* gcc.dg/maxof-compile.c: New test.
	* gcc.dg/maxof-pedantic-errors.c: New test.
	* gcc.dg/maxof-pedantic.c: New test.

Signed-off-by: Alejandro Colomar <alx@kernel.org>