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

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

In general, I think that if a memory map is involved, then it makes sense to flush the data. I don't think that the default behaviour is to do write through for performance reasons.

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