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

Linux CI bots take a bit over 70 seconds to fully set their git repos
up, always starting from scratch. This can end up being a significant
percentage of their overall runtime, depending on the change they're
vetting.

Having a fresh llvm-project in the docker image can bring this down to
about 15 seconds; if the llvm-project checkout is 5 weeks old, the
number seems to be closer to 30-35 seconds.

In any case, that's a meaningful savings per CI bot run, so include a
pinned LLVM here.

Author: gburgessiv

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

@@ -94,3 +94,14 @@ 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 ~800MB (4.1GB -> 3.3GB). 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