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: |

.github/workflows/test-llvm-builder.yml

@@ -4,11 +4,8 @@ on:
     branches: ["main"]
     types: [opened, synchronize]
     paths:
-      - 'LLVM.lock'
       - 'crates/llvm-builder/**'
       - '.github/workflows/test-llvm-builder.yml'
-    paths-ignore:
-      - "**.md"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
@@ -25,6 +22,8 @@ jobs:
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: true
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           # without this it will override our rust flags