Fetch LLVM release according to branch in llvm submodule (#418)

This change is essential first step to unblock gracefully enable LLVM 21
in revive compiler as current action blindly fetched latest LLVM binary
from the list of releases on our page.
The change queries release version from the submodule and uses it. Next
step: build LLVM 21 release binary
Next next step: switch revive compile to LLVM 21 after all issues are
fixed

Author: kvpanch

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

@@ -7,23 +7,56 @@ name: "Download LLVM"
 inputs:
   target:
     required: true
+  version:
+    description: "LLVM release version (e.g., 'llvm-18.1.8'). If not specified, uses the latest LLVM release."
+    required: false
+    default: ""
 
 runs:
   using: "composite"
   steps:
+    - name: detect llvm version from submodule
+      id: detect-version
+      shell: bash
+      run: |
+        if [ -z "${{ inputs.version }}" ]; then
+          # Extract branch from .gitmodules (e.g., "release/18.x")
+          BRANCH=$(git config -f .gitmodules submodule.llvm.branch)
+          if [ -n "$BRANCH" ]; then
+            # Extract version from branch name (e.g., "18.x" from "release/18.x")
+            VERSION_PREFIX=$(echo "$BRANCH" | sed 's|release/||' | sed 's|\.x$||')
+            echo "Detected LLVM version prefix from submodule branch: $VERSION_PREFIX"
+            
+            # Special case: pin LLVM 18 to specific version 18.1.8
+            if [ "$VERSION_PREFIX" = "18" ]; then
+              echo "Using pinned version for LLVM 18: llvm-18.1.8"
+              echo "version_prefix=llvm-18.1.8" >> $GITHUB_OUTPUT
+            else
+              echo "version_prefix=llvm-$VERSION_PREFIX" >> $GITHUB_OUTPUT
+            fi
+          else
+            echo "No branch found in .gitmodules, will use latest release"
+            echo "version_prefix=" >> $GITHUB_OUTPUT
+          fi
+        else
+          echo "Using explicitly provided version: ${{ inputs.version }}"
+          echo "version_prefix=${{ inputs.version }}" >> $GITHUB_OUTPUT
+        fi
+    
     - name: find asset
       id: find
       uses: actions/github-script@v7
       env:
         target: ${{ inputs.target }}
+        version_prefix: ${{ steps.detect-version.outputs.version_prefix }}
       with:
         result-encoding: string
         script: |
           let page = 1;
           let releases = [];
 
-          let releasePrefix = "llvm-"
           let target = process.env.target
+          let versionPrefix = process.env.version_prefix
 
           do {
             const res = await github.rest.repos.listReleases({
@@ -38,15 +71,31 @@ runs:
               return (a.published_at < b.published_at) ? 1 : ((a.published_at > b.published_at) ? -1 : 0);
             });
 
-            let llvmLatestRelease = releases.find(release => {
-              return release.tag_name.startsWith(releasePrefix);
-            });
-            if (llvmLatestRelease){
-                let asset = llvmLatestRelease.assets.find(asset =>{
+            let llvmRelease;
+            if (versionPrefix) {
+              // Search for latest release matching the version prefix
+              llvmRelease = releases.find(release => {
+                return release.tag_name.startsWith(versionPrefix);
+              });
+              if (llvmRelease) {
+                core.info(`Found LLVM release matching prefix '${versionPrefix}': ${llvmRelease.tag_name}`);
+              }
+            } else {
+              // Find latest LLVM release
+              llvmRelease = releases.find(release => {
+                return release.tag_name.startsWith('llvm-');
+              });
+              if (llvmRelease) {
+                core.info(`Found latest LLVM version: ${llvmRelease.tag_name}`);
+              }
+            }
+
+            if (llvmRelease){
+                let asset = llvmRelease.assets.find(asset =>{
                     return asset.name.includes(target);
                 });
                 if (!asset){
-                  core.setFailed(`Artifact for '${target}' not found in release ${llvmLatestRelease.tag_name} (${llvmLatestRelease.html_url})`);
+                  core.setFailed(`Artifact for '${target}' not found in release ${llvmRelease.tag_name} (${llvmRelease.html_url})`);
                   process.exit();
                 }
                 return asset.browser_download_url;
@@ -55,7 +104,11 @@ runs:
             page++;
           } while(releases.length > 0);
 
-          core.setFailed(`No LLVM releases with '${releasePrefix}' atifacts found! Please release LLVM before running this workflow.`);
+          if (versionPrefix) {
+            core.setFailed(`No LLVM releases matching prefix '${versionPrefix}' found! Please check the version.`);
+          } else {
+            core.setFailed(`No LLVM releases found! Please release LLVM before running this workflow.`);
+          }
           process.exit();
 
     - name: download