LLVM21 support

[felixaszx]

Will there be a LLVM21 base release soon?

[emaxx-google]

C-Vise tip-of-the-tree should be compatible with LLVM21 (per #183, #242), but there are several other ongoing changes that we should polish and complete before we call it an official release. The plan is to wrap them up within a couple of weeks.