libstdc++: Fix uses_allocator_construction args for cv pair (LWG 3677)

libstdc++: Fix uses_allocator_construction args for cv pair (LWG 3677)

The _Std_pair concept uses in <bits/uses_allocator_args.h> handles const
qualified pairs, but not volatile qualified. That's because it just uses
__is_pair which is specialized for const pairs.

This removes the partial specialization __is_pair<const pair<T,U>>, so
that __is_pair is now only true for cv-unqualified pairs. Then _Std_pair
needs to explicitly use remove_cv_t for the argument to __is_pair.

The other use of __is_pair is in map::insert(Pair&&) which doesn't want
to handle volatile so should just use remove_const_t.

libstdc++-v3/ChangeLog:

	* include/bits/stl_map.h (map::insert(Pair&&)): Use
	remove_const_t on argument to __is_pair.
	* include/bits/stl_pair.h (__is_pair<const pair<T,U>>): Remove
	partial specialization.
	* include/bits/uses_allocator_args.h (_Std_pair): Use
	remove_cv_t as per LWG 3677.
	* testsuite/20_util/uses_allocator/lwg3677.cc: New test.