libstdc++: Do not cast away const-ness in std::construct_at (LWG 3870)

This change also requires implementing the proposed resolution of LWG
3216 so that std::make_shared and std::allocate_shared still work, and
the proposed resolution of LWG 3891 so that std::expected still works.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h: Remove cv-qualifiers from
	type managed by _Sp_counted_ptr_inplace, as per LWG 3210.
	* include/bits/stl_construct.h: Do not cast away cv-qualifiers
	when passing pointer to placement new.
	* include/std/expected: Use remove_cv_t for union member, as per
	LWG 3891.
	* testsuite/20_util/allocator/void.cc: Do not test construction
	via const pointer.