libstdc++: Fix std::chrono::file_clock conversions for low-precision times

THe std::chrono::file_clock conversions were not using common_type and
so failed to compile when converting anything that should have increased
precision after arithmetic with a std::chrono::seconds value.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (__file_clock::from_sys)
	(__file_clock::to_sys, __file_clock::_S_from_sys)
	(__file_clock::_S_to_sys): Use common_type for return type.
	* testsuite/std/time/clock/file/members.cc: Check round trip
	conversion for time with lower precision that seconds.