The changes to "relocate" std::vector elements can lead to new errors
outside the immediate context, because moving the elements to new
storage no longer makes use of the move-if-noexcept utilities. This
means that types with deleted moves no longer degenerate to copies, but
are just ill-formed. The errors happen while instantiating the
noexcept-specifier for __relocate_object_a, when deciding whether to try
to relocate.

This patch introduces indirections to avoid the ill-formed
instantiations of std::__relocate_object_a. In order to avoid using
if-constexpr prior to C++17 this is done by tag dispatching. After this
patch all uses of std::__relocate_a are guarded by checks that will
support sensible code (i.e. code not using custom allocators that fool
the new checks).

	PR libstdc++/89130
	* include/bits/alloc_traits.h (__is_copy_insertable_impl): Rename to
	__is_alloc_insertable_impl. Replace single type member with two
	members, one for each of copy and move insertable.
	(__is_move_insertable): New trait for internal use.
	* include/bits/stl_vector.h (vector::_S_nothrow_relocate(true_type))
	(vector::_S_nothrow_relocate(true_type)): New functions to
	conditionally check if __relocate_a can throw.
	(vector::_S_use_relocate()): Dispatch to _S_nothrow_relocate based
	on __is_move_insertable.
	(vector::_S_do_relocate): New overloaded functions to conditionally
	call __relocate_a.
	(vector::_S_relocate): New function that dispatches to _S_do_relocate
	based on _S_use_relocate.
	* include/bits/vector.tcc (vector::reserve, vector::_M_realloc_insert)
	(vector::_M_default_append): Call _S_relocate instead of __relocate_a.
	* testsuite/23_containers/vector/modifiers/push_back/89130.cc: New.

From-SVN: r268537