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

[Reid Kleckner via llvm-commits]

rnk wrote:

I'm pretty hesitant about the idea of flushing files to disk on shutdown. My understanding is that the vast majority of software improvements to filesystem performance are attributable to caching data in memory and deferring disk writes. I can't find a quotation, but I believe I picked this up from Ted T'so somewhere (maybe [here](https://thunk.org/tytso/blog/2009/03/15/dont-fear-the-fsync/)), but reading that again, I wonder if he's trying to make the opposite point.

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.

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