Provide filesystem::path overloads for file streams (LWG 2676, partial)

Provide filesystem::path overloads for file streams (LWG 2676, partial)

	* include/std/fstream (basic_filebuf::_If_path): New SFINAE helper.
	(basic_filebuf::open<Path>(const Path&, const ios_base::openmode&))
	(basic_ifstream<Path>(const Path&, const ios_base::openmode&))
	(basic_ifstream::open<Path>(const Path&, const ios_base::openmode&))
	(basic_ofstream<Path>(const Path&, const ios_base::openmode&))
	(basic_ofstream::open<Path>(const Path&, const ios_base::openmode&))
	(basic_fstream<Path>(const Path&, const ios_base::openmode&))
	(basic_fstream::open<Path>(const Path&, const ios_base::openmode&)):
	New constructors and member functions.
	* testsuite/27_io/basic_filebuf/open/char/path.cc: New test.
	* testsuite/27_io/basic_fstream/cons/char/path.cc: New test.
	* testsuite/27_io/basic_fstream/open/char/path.cc: New test.
	* testsuite/27_io/basic_ifstream/cons/char/path.cc: New test.
	* testsuite/27_io/basic_ifstream/open/char/path.cc: New test.
	* testsuite/27_io/basic_ofstream/cons/char/path.cc: New test.
	* testsuite/27_io/basic_ofstream/open/char/path.cc: New test.

From-SVN: r254148