libstdc++: Fix declarations of variable templates

libstdc++: Fix declarations of variable templates

This code is invalid and rejected by other compilers (see PR 92576).

	* include/bits/regex.h (ranges::__detail::__enable_view_impl): Fix
	declaration.
	* include/bits/stl_multiset.h (ranges::__detail::__enable_view_impl):
	Likewise.
	* include/bits/stl_set.h (ranges::__detail::__enable_view_impl):
	Likewise.
	* include/bits/unordered_set.h (ranges::__detail::__enable_view_impl):
	Likewise.
	* include/debug/multiset.h (ranges::__detail::__enable_view_impl):
	Likewise.
	* include/debug/set.h (ranges::__detail::__enable_view_impl): Likewise.
	* include/debug/unordered_set (ranges::__detail::__enable_view_impl):
	Likewise.

From-SVN: r278440