Fix rate-limited cloning of LLVM submodule (#569)

## Description

We started seeing rate-limit `429` errors when cloning LLVM (as seen in
e.g. [this failed
run](https://github.com/paritytech/revive/actions/runs/28924583524/job/85809154518)
and [this failed
run](https://github.com/paritytech/revive/actions/runs/28926259175/job/85821336465?pr=564)).
`llvm/llvm-project` now seems to be heavily rate-limited by GitHub.
(Authenticating the clone did not seem to have much effect.)

Even though our resolc build steps only do a treeless clone, all build
steps still did the same clone.

### Changes

* I've instead introduced a `.llvm-version` file that the `get-llvm`
action reads from, eliminating the network request all together.
* `.llvm-version` is meant to be kept in sync with the LLVM submodule
version we use. This is enforced by:
  * Introduced a `check-llvm-version.yml` workflow
* Runs once to check that the recorded version in `.llvm-version`
coincides with the version detected from the submodule.
* The submodule version detection *does* perform a network request as
before, but this is only run once now (when relevant files change),
rather than for all our resolc build jobs.
* Since the differential test jobs require checking out our
revive-differential-tests (rdt) submodule, those checkout steps cloned
not only rdt, but also LLVM. I've updated this to not clone LLVM at all.

.github/actions/get-llvm/action.yml

@@ -14,48 +14,34 @@ inputs:
 runs:
   using: "composite"
   steps:
-    - name: detect llvm version from submodule
-      id: detect-version
+    - name: check override version
+      id: override
       shell: bash
       env:
         # To test a draft LLVM release across every CI workflow, set this to the full
         # draft name (e.g. "llvm-21.1.8-revive.0f80912") and revert to "" before merging.
         OVERRIDE_VERSION: ""
+      run: echo "version=${OVERRIDE_VERSION}" | tee -a "$GITHUB_OUTPUT"
+
+    - name: resolve llvm version
+      id: resolve-version
+      if: ${{ steps.override.outputs.version == '' }}
+      shell: bash
       run: |
         set -euo pipefail
 
-        if [ -n "${OVERRIDE_VERSION}" ]; then
-          echo "Using override version: ${OVERRIDE_VERSION}"
-          echo "version_prefix=${OVERRIDE_VERSION}" >> "$GITHUB_OUTPUT"
-          exit 0
-        fi
-
-        LLVM_SHA=$(git ls-tree HEAD llvm | awk '{print $3}')
-        echo "LLVM_SHA: $LLVM_SHA"
-        LLVM_URL=$(git config -f .gitmodules submodule.llvm.url)
-        echo "LLVM_URL: $LLVM_URL"
-        if [ -z "$LLVM_SHA" ] || [ -z "$LLVM_URL" ]; then
-          echo "::error::Could not resolve LLVM submodule SHA or URL from this repo." >&2
+        if [ ! -s .llvm-version ]; then
+          echo "::error file=.llvm-version::Missing or empty .llvm-version. It must contain the version matching the llvm submodule pin."
           exit 1
         fi
-
-        # Do a treeless clone to avoid unnecessarily fetching source files.
-        git clone --filter=tree:0 --no-checkout "$LLVM_URL" llvm-tmp
-        # Try to get the most recent tag (e.g., "llvmorg-18.1.8").
-        LLVM_TAG=$(git -C llvm-tmp describe --tags --abbrev=0 "$LLVM_SHA")
-        rm -rf llvm-tmp
-
-        # Convert "llvmorg-x.y.z" to "llvm-x.y.z".
-        LLVM_VERSION=$(echo "$LLVM_TAG" | sed 's/^llvmorg-/llvm-/')
-        echo "Detected LLVM version from submodule pin: $LLVM_VERSION"
-        echo "version_prefix=$LLVM_VERSION" >> "$GITHUB_OUTPUT"
+        echo "version_prefix=$(cat .llvm-version)" | tee -a "$GITHUB_OUTPUT"
 
     - name: find asset
       id: find
       uses: actions/github-script@v7
       env:
         target: ${{ inputs.target }}
-        version_prefix: ${{ steps.detect-version.outputs.version_prefix }}
+        version_prefix: ${{ steps.override.outputs.version || steps.resolve-version.outputs.version_prefix }}
       with:
         result-encoding: string
         script: |