[FileSystem] Ignore nanoseconds when comparing oso_mod_time

After a recent change in LLVM the TimePoint encoding become more
precise, exceeding the precision of the TimePoint obtained from the
DebugMap. This patch adds a flag to the GetModificationTime helper in
the FileSystem to return the modification time with less precision.

Thanks to Davide for bisecting this failure on the LLDB bots.

llvm-svn: 347615