c++: Implement __remove_extent built-in trait

c++: Implement __remove_extent built-in trait

This patch implements built-in trait for std::remove_extent.

gcc/cp/ChangeLog:

	* cp-trait.def: Define __remove_extent.
	* semantics.cc (finish_trait_type): Handle CPTK_REMOVE_EXTENT.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Test existence of __remove_extent.
	* g++.dg/ext/remove_extent.C: New test.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Jason Merrill <jason@redhat.com>