`<chrono>`: `file_clock` also needs leap second handling

[Observed](https://github.com/microsoft/STL/pull/1870#discussion_r617522208) by @statementreply

As of 2021-04-21, there will be no leap second counted in `file_clock` by 2021-12-30, and _likely_ none by 2022-06-29.  We need to investigate this further.  

Related to #1822.

Discovered while implementing #12.