c++: Implement __add_lvalue_reference built-in trait

c++: Implement __add_lvalue_reference built-in trait

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

gcc/cp/ChangeLog:

	* cp-trait.def: Define __add_lvalue_reference.
	* semantics.cc (finish_trait_type): Handle
	CPTK_ADD_LVALUE_REFERENCE.

gcc/testsuite/ChangeLog:

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

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