add LLVM 18.x as a git submodule (#399)

This changeset is based on https://github.com/paritytech/revive/pull/346

Makefile

@@ -38,7 +38,7 @@ install-llvm-builder:
 	cargo install --force --locked --path crates/llvm-builder
 
 install-llvm: install-llvm-builder
-	revive-llvm clone
+	git submodule update --init --recursive --depth 1
 	revive-llvm build --llvm-projects lld --llvm-projects clang
 
 install-revive-runner: