[Support] Fix 'keeping' temporary files on Windows 7

As reported here: https://bugs.llvm.org/show_bug.cgi?id=48378#c0
and here: rust-lang/rust#81051
since 79657e2, some programs such as llvm-ar
don't work properly on Windows 7.

The issue is shown in the snippet by Oleksandr Prodan:
https://pastebin.com/v51m3uBU

In essence, once the 'DeleteFile' flag has been set on FILE_DISPOSITION_INFO,
the file path can't be queried anymore with GetFinalPathNameByHandleW. This
however works on Windows 10, GetFinalPathNameByHandleW would return sucessfully.

To workaround the issue, we simply reset the 'DeleteFile' flag before even
checking if we're dealing with a network file.

Tested with `llvm-ar r empty.a a.obj` ran on a network mount. At the moment, we
cannot specifically add a test coverage for this, since it requres mounting a
network drive.

llvm/lib/Support/Windows/Path.inc

@@ -402,8 +402,22 @@ std::error_code is_local(int FD, bool &Result) {
 }
 
 static std::error_code setDeleteDisposition(HANDLE Handle, bool Delete) {
-  // First, check if the file is on a network (non-local) drive. If so, don't
-  // set DeleteFile to true, since it prevents opening the file for writes.
+  // Clear the FILE_DISPOSITION_INFO flag first, before checking if it's a
+  // network file. On Windows 7 the function realPathFromHandle() below fails
+  // if the FILE_DISPOSITION_INFO flag was already set to 'DeleteFile = true' by
+  // a prior call.
+  FILE_DISPOSITION_INFO Disposition;
+  Disposition.DeleteFile = false;
+  if (!SetFileInformationByHandle(Handle, FileDispositionInfo, &Disposition,
+                                  sizeof(Disposition)))
+    return mapWindowsError(::GetLastError());
+  if (!Delete)
+    return std::error_code();
+
+  // Check if the file is on a network (non-local) drive. If so, don't
+  // continue when DeleteFile is true, since it prevents opening the file for
+  // writes. Note -- this will leak temporary files on disk, but only when the
+  // target file is on a network drive.
   SmallVector<wchar_t, 128> FinalPath;
   if (std::error_code EC = realPathFromHandle(Handle, FinalPath))
     return EC;
@@ -415,9 +429,9 @@ static std::error_code setDeleteDisposition(HANDLE Handle, bool Delete) {
   if (!IsLocal)
     return std::error_code();
 
-  // The file is on a local drive, set the DeleteFile to true.
-  FILE_DISPOSITION_INFO Disposition;
-  Disposition.DeleteFile = Delete;
+  // The file is on a local drive, we can safely set FILE_DISPOSITION_INFO's
+  // flag.
+  Disposition.DeleteFile = true;
   if (!SetFileInformationByHandle(Handle, FileDispositionInfo, &Disposition,
                                   sizeof(Disposition)))
     return mapWindowsError(::GetLastError());