Add support for LLVM 18

[nathanchance]

• Update LLVM main anchor and version to 19 (pull request)
• Wait for LLVM 18 binaries on apt.llvm.org
• Wait for TuxMake to provide clang-18 images (done)
• Wait for TuxSuite to allow clang-18 to be specified
• Update generator.yml to provide llvm_17 anchor and copy all llvm_latest builds to it
• Update generator.yml to shift llvm_latest to 18 and copy all llvm_tot builds to it

[nathanchance]

Done as of #704.