libstdc++: Fix experimental::path::generic_string (PR 93245)

libstdc++: Fix experimental::path::generic_string (PR 93245)

This function was unimplemented, simply returning the native format
string instead.

	PR libstdc++/93245
	* include/experimental/bits/fs_path.h (path::generic_string<C,T,A>()):
	Return the generic format path, not the native one.
	* testsuite/experimental/filesystem/path/generic/generic_string.cc:
	Improve test coverage.

(cherry picked from commit a577c0c26931090e7c25e56ef5ffc807627961ec)