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

[Reid Kleckner via llvm-commits]

rnk wrote:

> We had the same discussion on [this](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?

I agree that the flag is not a long term solution for the users affected by the virtualbox issue. It would be a way to commit the code change, allow unaffected users to measure the cost of additional flushing with low effort (flag flips instead of local patches), and then we can refine the auto-detection logic for which devices require the flush, or decide how important it is to have a narrow criteria.

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