[github] pre-cache llvm-project in Linux CI Docker images

Linux CI bots take a bit over a minute to fully set their git repos up,
always starting from scratch. This can end up being a significant
percentage of their overall runtime.

If llvm-project is built into the images & used as a cache, we could
potentially speed that up quite a bit. Try it out and see.

Author: gburgessiv

.github/workflows/containers/github-action-ci/Dockerfile

@@ -103,3 +103,14 @@ RUN mkdir actions-runner && \
     tar xzf ./actions-runner-linux-x64-$GITHUB_RUNNER_VERSION.tar.gz && \
     rm ./actions-runner-linux-x64-$GITHUB_RUNNER_VERSION.tar.gz
 
+# Pre-cache llvm-project in these images; this works with the premerge action
+# to speed up runs; cloning LLVM fresh takes >1min otherwise.
+#
+# A local experiment showed an explicit `git gc --aggressive` reduced this
+# layer's size by 20% (~800MB). Disable GC'ing afterward, so we don't waste time
+# trying to do that in CI.
+RUN \
+  git clone https://github.com/llvm/llvm-project --tags && \
+  cd llvm-project && \
+  git gc --aggressive && \
+  git config gc.auto 0

.github/workflows/premerge.yaml

@@ -26,6 +26,11 @@ concurrency:
 jobs:
   premerge-checks-linux:
     name: Linux Premerge Checks (Test Only - Please Ignore Results)
+    defaults:
+      run:
+        # The premerge Linux docker instance keeps a pristine clone of LLVM in
+        # /home/gha/llvm-project; that should be used as our main workdir.
+        working-directory: /home/gha/llvm-project
     if: >-
         github.repository_owner == 'llvm' &&
         (github.event_name != 'pull_request' || github.event.action != 'closed')
@@ -34,7 +39,7 @@ jobs:
       - name: Checkout LLVM
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          fetch-depth: 2
+          path: /home/gha/llvm-project
       - name: Setup ccache
         uses: hendrikmuhs/ccache-action@a1209f81afb8c005c13b4296c32e363431bffea5 # v1.2.17
         with: