Define new filesystem::__file_clock type

Define new filesystem::__file_clock type

In C++17 the clock used for filesystem::file_time_type is unspecified,
allowing it to be chrono::system_clock. The C++2a draft requires it to
be a distinct type, with additional member functions to convert to/from
other clocks (either the system clock or UTC). In order to avoid an ABI
change later, this patch defines a new distinct type now, which will be
used for std::chrono::file_clock later.

	* include/bits/fs_fwd.h (__file_clock): Define new clock.
	(file_time_type): Redefine in terms of __file_clock.
	* src/filesystem/ops-common.h (file_time): Add FIXME comment about
	overflow.
	* src/filesystem/std-ops.cc (is_set(perm_options, perm_options)): Give
	internal linkage.
	(internal_file_lock): New helper type for accessing __file_clock.
	(do_copy_file): Use internal_file_lock to convert system time to
	file_time_type.
	(last_write_time(const path&, error_code&)): Likewise.
	(last_write_time(const path&, file_time_type, error_code&)): Likewise.

From-SVN: r267602