Bug 40249: Bump mingw-w64 and clang for Firefox 78.9
This is the same as !240 (closed), with fixup commits squashed.
I was able to build mingw-w64-clang on Windows i686 and x86_64, and rust on Windows x86_64 (I am still building it on i686).
I did not test building Firefox 78.9 yet.
Closes #40249 (closed).