[libcxx] Fix last_write_time test for filesystems that don't support very small times.

APFS minimum supported file write time is -2^63 nanoseconds, which doesn't go
as far as `file_time_type::min()` that is equal to -2^63 microseconds on macOS.

This change doesn't affect filesystems that support `file_time_type` range only
for in-memory file time representation but not for on-disk representation. Such
filesystems are considered as `SupportsMinTime`.

rdar://problem/35865151

Reviewers: EricWF, Hahnfeld

Subscribers: jkorous-apple, mclow.lists, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D42755

llvm-svn: 326383