Infer full LLVM release version from git's tags (#437)

The helps to remove special casing from actions and will make future
LLVM updates smoother.

Author: kvpanch

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

@@ -20,29 +20,27 @@ runs:
       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" >> $GITHUB_OUTPUT
-            fi
+          # Get the full version from the LLVM submodule.
+          git submodule update --init --recursive
+          cd llvm
+          # Try to get the most recent tag (e.g., "llvmorg-18.1.8")
+          LLVM_TAG=$(git describe --tags --abbrev=0 2>/dev/null)
+          if [ -n "$LLVM_TAG" ]; then
+            echo "Detected LLVM version from submodule: $LLVM_TAG"
+            # Convert "llvmorg-x.y.z" to "llvm-x.y.z"
+            LLVM_VERSION=$(echo "$LLVM_TAG" | sed 's/^llvmorg-/llvm-/')
+            echo "Detected LLVM version from submodule: $LLVM_VERSION"
+            echo "version_prefix=$LLVM_VERSION" >> $GITHUB_OUTPUT
           else
-            echo "No branch found in .gitmodules, will use latest release"
+            echo "Could not detect LLVM version from submodule, will use latest release"
             echo "version_prefix=" >> $GITHUB_OUTPUT
           fi
+          cd ..
         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