libstdc++: Fix common_reference for non-reference results [PR100894]

libstdc++: Fix common_reference for non-reference results [PR100894]

The result of COMMON-REF(A&, B&&) where they have no common reference
type should be ill-formed. Our implementation fails to check that the
COMMON-REF result is a reference, so is well-formed when it shouldn't
be. This means that common_reference uses that result when it shouldn't
do.

The fix is to reject the result of COMMON-REF(A, B) if it's not a
reference, so that common_reference falls through to the next case,
which uses COND-RES, which yields the correct non-reference result.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/100894
	* include/std/type_traits (__common_ref_impl<X&, Y&>): Only
	use the type if it's a reference.
	* testsuite/20_util/common_reference/100894.cc: New test.

(cherry picked from commit c37b5ddcc88e0cc0f6a4ad609eda51021df0f6bb)