PR libstdc++/83025 fix constraints for path overloads in <fstream>

	PR libstdc++/83025
	* include/std/fstream (basic_filebuf::_If_path): Move to
	namespace-scope and rename to _If_fs_path.
	(basic_filebuf::open): Use new name.
	(basic_ifstream(_Path, ios::openmode))
	(basic_ifstream::open(_Path, ios::openmode))
	(basic_ofstream(_Path, ios::openmode))
	(basic_ofstream::open(_Path, ios::openmode))
	(basic_fstream(_Path, ios::openmode))
	(basic_fstream::open(_Path, ios::openmode)): Use _If_fs_path.
	* testsuite/27_io/basic_filebuf/open/char/path.cc: Test with filename
	as non-const char*.
	* testsuite/27_io/basic_fstream/cons/char/path.cc: Likewise.
	* testsuite/27_io/basic_fstream/open/char/path.cc: Likewise.
	* testsuite/27_io/basic_ifstream/cons/char/path.cc: Likewise.
	* testsuite/27_io/basic_ifstream/open/char/path.cc: Likewise.
	* testsuite/27_io/basic_ofstream/cons/char/path.cc: Likewise.
	* testsuite/27_io/basic_ofstream/open/char/path.cc: Likewise.

From-SVN: r254871