A couple cases popped up where we were using an undefined cast.
Fix them by rearranging assignment macros to use decltype.

Another warning that is enabled in Skia.

Bug: angleproject:4046
Change-Id: Ibdbae0b2a96909b101a259ff805b8b2d43a5174b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1877479
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>