Add design docs for upcoming file_time_type change.

In upcoming changes to filesystem I plan to change file_time_type
to use __int128_t as its underlying representation, in order
to allow it to have a range and resolution at least that of
the timespec struct.

There was some pushback against this decision, so I drafted
a document explaining the problems, potential solutions, and
the rational for the decision.

However, it's probably easier to let people read the generated
HTML rather than the raw restructured text. For this reason
I'm commiting the design documents before hand, so they can
be available during any subsequent discussion or code review.

llvm-svn: 337880

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