This switches clang to be the default compiler on Windows ARM64, allowing for an override to MSVC.
Turns out MSVC builds have been broken for months, but nobody checked, so I'm just switching them off for now and setting clang as the default.
These updated scripts allow for the msbuild generator to use an external (ie, non-MSVC) clang installation properly, otherwise they failed.
They also allow for users to specify their own desired clang compiler via an environment variable.
An update to the docs will come seperately.
Pull Request: https://projects.blender.org/blender/blender/pulls/134566
d29ef95c3c