libstdc++: Improve overflow check for file timestamps

libstdc++: Improve overflow check for file timestamps

The current code assumes that system_clock::duration is nanoseconds, and
also performs a value-changing conversion from nanoseconds::max() to
double (which doesn't matter after dividing by 1e9, but triggers a
warning with Clang nonetheless).

A better solution is to use system_clock::duration::max() and perform
the comparison entirely using the std::chrono types, rather than with
dimensionless arithmetic types.

This doesn't address the FIXME in the function, so the overflow check
still rejects some values that could be represented by the file_clock.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* src/filesystem/ops-common.h (filesystem::file_time): Improve
	overflow check by using system_clock::duration::max().