libstdc++: Optimize std::add_lvalue_reference compilation performance

libstdc++: Optimize std::add_lvalue_reference compilation performance

This patch optimizes the compilation performance of
std::add_lvalue_reference by dispatching to the new
__add_lvalue_reference built-in trait.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__add_lval_ref_t): Use
	__add_lvalue_reference built-in trait.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>