[llvm] [Support] Always call FlushFileBuffers() when unmapping memory on Windows (PR #78597)

[Jacek Caban via llvm-commits]

cjacek wrote:

I think that instead of `FlushFileBuffers`, we could use `FlushViewOfFile` before unmapping. In theory it should be cheaper and doing that with your test case fixes the problem on VirtualBox for me (and hopefully the kernel bug too).

https://github.com/llvm/llvm-project/pull/78597