[llvm] Avoid calling `GetFileAttributesW` in Windows' `fs::access` when checking for existence (PR #83495)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> There is a race condition when calling `GetFileAttributesW` that can cause it to return `ERROR_ACCESS_DENIED` on a path which exists

Can you elaborate a bit more about how this race condition occurs, and how the suggested solution avoids the issue? (I guess it's explained somewhere in the linked issues, but in order to evaluate the suggested fix, it would be good to have that analysis spelled out here as well.)

https://github.com/llvm/llvm-project/pull/83495