Setup Gtihub Action to build LLVM

Author: miry

.github/workflows/llvm.yml

@@ -0,0 +1,28 @@
+name: Darwin LLVM build
+
+on: [push, pull_request]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  LLVM_VERSION: 15.0.7
+  MACOSX_DEPLOYMENT_TARGET: 10.11
+
+jobs:
+  llvm-build:
+    runs-on: macos-13
+    steps:
+      - name: Download Crystal source
+        uses: actions/checkout@v4
+
+      - name: Prepare folders
+        run: |
+          brew update
+          brew install pkgconfig libtool cmake
+
+          sudo mkdir -p /opt/llvm
+          sudo chown $(whoami) /opt/llvm/
+          sudo mkdir -p /var/cache
+          sudo chown $(whoami) /var/cache