Implement P0504R0 (Revisiting in-place tag types for
any/optional/variant).
* include/std/any (any(_ValueType&& __value)): Constrain
the __is_in_place_type with the decayed type.
(make_any): Adjust to use the new tag type.
* include/std/utility (in_place_tag): Remove.
(in_place_t): Turn into a non-reference tag type.
(__in_place, __in_place_type, __in_place_index): Remove.
(in_place): Turn into an inline variable of non-reference
tag type.
(in_place<_Tp>): Remove.
(in_place_index<_Idx>): Remove.
(in_place_type_t): New.
(in_place_type): Turn into a variable template of non-reference
type.
(in_place_index_t): New.
(in_place_index): Turn into a variable template of non-reference
type.
* include/std/variant
(_Variant_storage(in_place_index_t<_Np>, _Args&&...)): Adjust to
use the new tag type.
(_Union(in_place_index_t<0>, _Args&&...)): Likewise.
(_Union(in_place_index_t<_Np>, _Args&&...)): Likewise.
(_Variant_base()): Likewise.
(variant(_Tp&&)): Likewise.
(variant(in_place_type_t<_Tp>, _Args&&...)): Likewise.
(variant(in_place_type_t<_Tp>, initializer_list<_Up>,
_Args&&...)): Likewise.
(variant(in_place_index_t<_Np>, _Args&&...)): Likewise.
(variant(in_place_index_t<_Np>, initializer_list<_Up>,
_Args&&...)): Likewise
(variant(allocator_arg_t, const _Alloc&)): Likewise.
(variant(allocator_arg_t, const _Alloc&, _Tp&&)): Likewise.
(variant(allocator_arg_t, const _Alloc&, in_place_type_t<_Tp>,
_Args&&...)): Likewise.
(variant(allocator_arg_t, const _Alloc&, in_place_type_t<_Tp>,
initializer_list<_Up>, _Args&&...)): Likewise.
(variant(allocator_arg_t, const _Alloc&, in_place_index_t<_Np>,
_Args&&...)): Likewise.
(variant(allocator_arg_t, const _Alloc&, in_place_index_t<_Np>,
initializer_list<_Up>, _Args&&...)): Likewise.
(emplace(_Args&&...)): Likewise.
(emplace(initializer_list<_Up>, _Args&&...)): Likewise.
* testsuite/20_util/any/cons/explicit.cc: Likewise.
* testsuite/20_util/any/cons/in_place.cc: Likewise.
* testsuite/20_util/any/requirements.cc: Add tests to
check that any is not constructible from the new in_place_type_t
of any value category.
* testsuite/20_util/in_place/requirements.cc: Adjust to
use the new tag type.
* testsuite/20_util/variant/compile.cc: Likewise.
* testsuite/20_util/variant/run.cc: Likewise.

From-SVN: r242401