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

[Alexandre Ganea via llvm-commits]

aganea wrote:

> Maybe the way forward here is to land the flush operation under a flag so folks can measure performance in different scenarios, since it doesn't seem reasonable to put the burden of proving there is no performance regression on Martin.

We had the same discussion on [the ](https://reviews.llvm.org/D155579). I'd prefer auto-detection of the problematic target drive because new flags are not easily discoverable by users and we will end with this same bug report. But if that's not possible we can also do a flag. In that case what about making things correct by default and a flag for disabling the flush?

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