[llvm] [Support] Avoid a VirtualBox shared folders mmap bug (PR #78597)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I updated the patch to call `is_local`, and do a flush only if that indicates that the file system isn't local, unless we're flushing to avoid the kernel bug anyway.

For the rare kernel bug case, we don't know if calling `FlushViewOfFile` instead of `FlushFileBuffers` works. (It probably does, but verifying it is nontrivial.) Therefore, we probably don't want to change that case. For the new heuristic case, I kept calling `FlushFileBuffers` just for code simplicity reasons, to allow using the same codepath for both cases, because the performance difference between the two flush alternatives seems to be very small in practice.

This keeps the usual much faster performance for writing large PDB files on normal storage, while fixing the correctness in the VirtualBox case.

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