Remove TimeValue usage from FileSpec.h

Summary:
The only usage there was in GetModificationTime(). I also took the opportunity
to move this function from FileSpec to the FileSystem class - since we are
using FileSpecs to also represent remote files for which we cannot (easily)
retrieve modification time, it makes sense to make the decision to get the
modification time more explicit.

The new function returns a llvm::sys::TimePoint<>. To aid the transition
from TimeValue, I have added a constructor to it which enables implicit
conversion from a time_point.

Reviewers: zturner, clayborg

Subscribers: mehdi_amini, tberghammer, danalbert, beanz, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D25392

llvm-svn: 285702