Fixes https://github.com/llvm/llvm-project/issues/83046
There is a race condition when calling `GetFileAttributesW` that can
cause it to return `ERROR_ACCESS_DENIED` on a path which exists, which
is unexpected for callers using this function to check for file
existence by passing `AccessMode::Exist`. This was manifesting as a
compiler crash on Windows downstream in the Swift compiler when using
the `-index-store-path` flag (more information in
https://github.com/apple/llvm-project/issues/8224).
I looked for alternate APIs to avoid bringing in `shlwapi.h`, but didn't
see any good candidates. I'm not tied at all to this solution, any
feedback and alternative approaches are more than welcome.
9961c03e9e