Build LLVM from source in CI

We build Alloy in CI, which requires us to have LLVM installed. Rust
no longer provides LLVM binaries for our rustc version so we must build
it ourselves.

.buildbot.config.toml

@@ -10,6 +10,7 @@ extended = false
 
 [llvm]
 assertions = true           # Turn on assertions in LLVM.
+download-ci-llvm = false
 
 [install]
 prefix = "build/rustc_boehm"