This switches the generator script use the hermetic Clang instead
of the system gcc/clang. It also uses common Python routines to
manage the temporary file so that it works consistently on Win.

Bug: angleproject:5186
Change-Id: I52906d1a708db8b925061a9d5578b3d54a6dc862
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2483464Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>