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

[Alexandre Ganea via llvm-commits]

aganea wrote:

@mstorsjo Are you able to test linking a large PDB by chance? 4GB+ PDB are becoming quite common nowadays, and since the PDB isn’t needed just after link, the previous optimization I did was saving several seconds of link time. Corectness is needed before performance, but this PR would impact cases which don’t need the flush (I assume most of LLD usages on Windows). Can we rather detect problematic cases on LLD startup, like writing to a virtual/network drive, and only disable the optimization there?

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