Deprecate nested types in std::hash

Deprecate nested types in std::hash

	* include/bits/c++config (_GLIBCXX17_DEPRECATED): Define.
	* include/bits/functional_hash.h (__hash_base::result_type)
	(__hash_base::argument_type): Add _GLIBCXX17_DEPRECATED.
	* include/std/optional (hash<optional<T>>::result_type)
	(hash<optional<T>>::argument_type): Add deprecated attribute.
	(__is_fast_hash<hash<optional<T>>>): Add partial specialization.
	* include/std/variant (hash<variant<Types...>>::result_type)
	(hash<variant<Types...>>::argument_type): Add deprecated attribute.
	(__is_fast_hash<hash<variant<Types...>>>): Add partial specialization.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@253017 138bc75d-0d04-0410-961f-82ee72b054a4