toolchains

Custom LLVM toolchains for Mull.

Each CI run publishes a release tagged with the workflow run id, with these assets per platform (Darwin-aarch64, Linux-aarch64, Linux-x86_64):

• <OS>-<ARCH>.toolchain.tar.xz: the LLVM toolchain (clang, lld, compiler-rt, tools), also including FileCheck. clang, lld and most LLVM tools are symlinks to a single multi-call bin/llvm binary. clang-format, clang-tidy and clangd are stubs that exit with an error: they're only there because toolchains_llvm expects them. There's no libc++, libunwind or libLTO, and compiler-rt has no libFuzzer, ORC, XRay or iOS runtimes.
• <OS>-<ARCH>.filecheck.tar.xz: just bin/FileCheck, for test suites that don't need the whole toolchain.

The LLVM revision is set by LLVM_REF in .github/workflows/toolchain.yml.