[Github] Add initial version of precommit checks

This patch adds an initial version of the LLVM precommit checks. These
checks should be reasonably performant (full test cycle in ~30 minutes
with a fully warm cache, ~80 minutes with a completely cold cache) and
should catch regressions within LLVM.

This is mainly intended to test the scalability of the current design
and to start eliminating issues before we begin to scale to other
subprojects.

Author: boomanaiden154

.github/workflows/precommit.yaml

@@ -0,0 +1,47 @@
+name: "Precommit tests"
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - '.github/workflows/precommit.yaml'
+      - 'llvm/**'
+
+jobs:
+  build-llvm:
+    name: "Build and test LLVM"
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/llvm/ci-ubuntu-22.04:latest
+    steps:
+      - name: Fetch LLVM sources
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+      - name: Setup ccache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          max-size: 500M
+          variant: sccache
+          key: precommit-linux
+      - name: Configure LLVM
+        run: |
+          cmake -B llvm-build -GNinja \
+            -DCMAKE_BUILD_TYPE=Release \
+            -DCMAKE_C_COMPILER_LAUNCHER=sccache \
+            -DCMAKE_CXX_COMPILER_LAUNCHER=sccache \
+            -DLLVM_ENABLE_ASSERTIONS=ON \
+            -DCMAKE_C_COMPILER=clang \
+            -DCMAKE_CXX_COMPILER=clang++ \
+            -DLLVM_LIT_ARGS="-v --no-progress-bar" \
+            ./llvm
+      - name: Build LLVM
+        run: |
+          ninja -C llvm-build llvm-test-depends
+      - name: Check LLVM
+        run: |
+          ninja -C llvm-build check-llvm