Add scheduled CI for StableHLO builds against LLVM@HEAD (#1562)

Felt inspired after seeing a presentation on JAX CI in
[openxla/openxla-pjrt-plugin](https://github.com/openxla/openxla-pjrt-plugin)
(like
[run_jaxtests_cpu.yml](https://github.com/openxla/openxla-pjrt-plugin/blob/main/.github/workflows/run_jaxtests_cpu.yml)),
decided to add some frequent (every 6hr) builds against the latest LLVM
revision to detect compatibility / other issues as soon as possible.

This is built on top of existing CI. The name of the buildAndTestCmake
job now includes what StableHLO is being built against. Examples from my
fork:
- [cmake-build
(llvm-project@HEAD)](https://github.com/GleasonK/stablehlo/actions/runs/5136120754/jobs/9242454311)
- [cmake-build
(llvm_version.txt)](https://github.com/GleasonK/stablehlo/actions/runs/5136234330/jobs/9242713655)

_Note: This only builds scheduled CI against HEAD, meaning it should
have no impact on CI for pull requests._

Other things to consider:
- If LLVM build fails (not StableHLO build) should we exit with success,
indicating a likely upstream breakage?
- How should these be monitored? Is subscribing to email notification
enough?

Closes #1489

.github/workflows/buildAndTestCMake.yml

@@ -20,8 +20,8 @@ on:
   push:
     branches: [ main ]
   schedule:
-    # Run once a day
-    - cron:  '0 12 * * *'
+    # Run once every 6hr building against llvm-project@HEAD
+    - cron:  '0 */6 * * *'
   workflow_dispatch:
 
 # Ensure that only a single job or workflow using the same
@@ -34,6 +34,7 @@ concurrency:
 
 jobs:
   cmake-build:
+    name: "cmake-build ${{ github.event_name == 'schedule' && '(llvm-project@HEAD)' || ''}}"
     env:
       LLVM_PROJECT_DIR: "llvm-project"
       LLVM_BUILD_DIR: "llvm-build"
@@ -51,7 +52,12 @@ jobs:
       id: llvm-version
       shell: bash
       run: |
-        echo "version=$(cat ${{ github.workspace }}/build_tools/llvm_version.txt)" >> $GITHUB_OUTPUT
+        USE_LLVM_HEAD=${{ github.event_name == 'schedule' }}
+        if [[ $USE_LLVM_HEAD = true ]]; then
+          echo "version=main" >> $GITHUB_OUTPUT
+        else
+          echo "version=$(cat ${{ github.workspace }}/build_tools/llvm_version.txt)" >> $GITHUB_OUTPUT
+        fi;
 
     - name: Setup workspace
       uses: ./.github/actions/setup-build