[llvm] [Support] Add mapped_file_region::sync(), equivalent to msync (PR #153632)

[Alexandre Ganea via llvm-commits]

aganea wrote:

> > Yeah I did change my example after posting it! Your example passes the test.
> 
> Thanks @aganea for the help! Updated.

You're very welcome! Let me know if you need further help for Windows.

https://github.com/llvm/llvm-project/pull/153632