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

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

.github/workflows/release-llvm.yml

@@ -96,10 +96,6 @@ jobs:
         run: |
           cargo install --locked --force --path crates/llvm-builder
 
-      - name: Clone LLVM
-        run: |
-          revive-llvm --target-env ${{ matrix.builder-arg }} clone
-
       - name: Build LLVM
         if: ${{ matrix.target != 'wasm32-unknown-emscripten' }}
         run: |