Support: Add mapped_file_region::sync(), equivalent to msync

Add mapped_file_region::sync(), equivalent to POSIX msync,
synchronizing written content to disk without unmapping the region.
Asserts if the mode is not mapped_file_region::readwrite.

Note that I don't have access to a Windows machine, so I can't
easily run those unit tests.

Change by dexonsmith

Differential Revision: https://reviews.llvm.org/D95494