Return errc::no_such_file_or_directory in fs::access if GetFileAttributesW fails (llvm#83495)

Fixes llvm#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
#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.

Author: z2oh

llvm/lib/Support/Windows/Path.inc

@@ -623,6 +623,10 @@ std::error_code access(const Twine &Path, AccessMode Mode) {
   DWORD Attributes = ::GetFileAttributesW(PathUtf16.begin());
 
   if (Attributes == INVALID_FILE_ATTRIBUTES) {
+    // Avoid returning unexpected error codes when querying for existence.
+    if (Mode == AccessMode::Exist)
+      return errc::no_such_file_or_directory;
+
     // See if the file didn't actually exist.
     DWORD LastError = ::GetLastError();
     if (LastError != ERROR_FILE_NOT_FOUND && LastError != ERROR_PATH_NOT_FOUND)