Fix bugs in last_write_time implementation.

* Fix passing a negative number as either tv_usec or tv_nsec. When file_time_type
  is negative and has a non-zero sub-second value we subtract 1 from tv_sec
  and make the sub-second duration positive.

* Detect and report when 'file_time_type' cannot be represented by time_t. This
  happens when using large/small file_time_type values with a 32 bit time_t.

There is more work to be done in the implementation. It should start to use
stat's st_mtim or st_mtimeval if it's provided as an extension. That way
we can provide a better resolution.

llvm-svn: 273103