Fix namespace versioning and remove __detail::__identity helpers

Fix namespace versioning and remove __detail::__identity helpers

	PR libstdc++/79243
	* include/bits/c++config (literals::string_view_literals::__7): Add.
	Only declare versioned namespaces for the relevant C++ dialects.
	* include/experimental/bits/erase_if.h (fundamentals_v2::__detail):
	Add versioning macros.
	* include/experimental/bits/lfts_config.h:
	(fundamentals_v1::__detail::__7, fundamentals_v2::__detail::__7): Add.
	* include/experimental/string_view (fundamentals_v2::__detail):
	Add versioning macros.
	(fundamentals_v2::__detail::__identity): Remove.
	(fundamentals_v2::__detail::__idt): Use common_type instead of
	__detail::__identity.
	* include/std/string_view (__detail::__identity, __detail::__idt):
	Likewise.
	(literals::string_view_literals): Fix nesting of versioning macros.

From-SVN: r244945