Add support for LLVM 20

[nathanchance]

• Update LLVM main anchor and version to 21 (pull request)
• Wait for LLVM 20 binaries on kernel.org
• Wait for TuxMake to provide korg-clang-20 images
• Wait for TuxSuite to allow korg-clang-20 to be specified
• Update generator/yml to provide llvm_19 anchor and copy all llvm_latest builds to it
• Update generator/yml to shift llvm_latest to 20 and copy all llvm_tot builds to it