libstdc++: Constrain std::vector default constructor [PR113841]

libstdc++: Constrain std::vector default constructor [PR113841]

This is needed to avoid errors outside the immediate context when
evaluating is_default_constructible_v<vector<T, A>> when A is not
default constructible.

To avoid diagnostic regressions for 23_containers/vector/48101_neg.cc we
need to make the std::allocator<cv T> partial specializations default
constructible, which they probably should have been anyway.

libstdc++-v3/ChangeLog:

	PR libstdc++/113841
	* include/bits/allocator.h (allocator<cv T>): Add default
	constructor to partial specializations for cv-qualified types.
	* include/bits/stl_vector.h (_Vector_impl::_Vector_impl()):
	Constrain so that it's only present if the allocator is default
	constructible.
	* include/bits/stl_bvector.h (_Bvector_impl::_Bvector_impl()):
	Likewise.
	* testsuite/23_containers/vector/cons/113841.cc: New test.