CI workflow to check clang-format usage on pull requests

Adapted but heavily copied from LLVM version of pr-code-format.yml.

.github/workflows/pr-code-format.yml

@@ -0,0 +1,98 @@
+# Inspired by LLVM's pr-code-format.yml at 
+# https://github.com/llvm/llvm-project/blob/main/.github/workflows/pr-code-format.yml
+
+name: "Check code formatting"
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  code_formatter:
+    runs-on: ubuntu-latest
+    if: github.repository == 'FEX-Emu/FEX'
+    steps:
+      - name: Fetch FEX sources
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+
+      - name: Checkout through merge base
+        uses: rmacklin/fetch-through-merge-base@v0
+        with:
+          base_ref: ${{ github.event.pull_request.base.ref }}
+          head_ref: ${{ github.event.pull_request.head.sha }}
+          deepen_length: 500
+
+      - name: Get changed files
+        id: changed-files
+        uses: tj-actions/changed-files@v39
+        with:
+          separator: ","
+          skip_initial_fetch: true
+
+      # We need to pull the script from the main branch, so that we ensure
+      # we get the latest version of this script.
+      - name: Fetch code formatting utils
+        uses: actions/checkout@v4
+        with:
+          repository: ${{ github.repository }}
+          ref: ${{ github.base_ref }}
+          sparse-checkout: |
+            llvm/utils/git/requirements_formatting.txt
+            llvm/utils/git/code-format-helper.py
+          sparse-checkout-cone-mode: false
+          path: code-format-tools
+
+      - name: "Listed files"
+        env:
+          CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
+        run: |
+          echo "Formatting files:"
+          echo "$CHANGED_FILES"
+
+      - name: Install clang-format
+        uses: aminya/setup-cpp@v1
+        with:
+          clangformat: 18.1.1
+
+      - name: Setup Python env
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.11'
+          cache: 'pip'
+          cache-dependency-path: 'code-format-tools/llvm/utils/git/requirements_formatting.txt'
+
+      - name: Install python dependencies
+        run: pip install -r code-format-tools/llvm/utils/git/requirements_formatting.txt
+
+      - name: Run code formatter
+        env:
+          GITHUB_PR_NUMBER: ${{ github.event.pull_request.number }}
+          START_REV: ${{ github.event.pull_request.base.sha }}
+          END_REV: ${{ github.event.pull_request.head.sha }}
+          CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
+        # TODO(boomanaiden154): Once clang v18 is released, we should be able
+        # to take advantage of the new --diff_from_common_commit option
+        # explicitly in code-format-helper.py and not have to diff starting at
+        # the merge base.
+        # Create an empty comments file so the pr-write job doesn't fail.
+        # (FEX) Actually v18 has been released but we don't want to fork the 
+        # files from the LLVM repo so we have to wait until code-format-helper.py
+        # is updated to support the new option.
+        run: |
+          echo "[]" > comments &&
+          python ./code-format-tools/llvm/utils/git/code-format-helper.py \
+            --write-comment-to-file \
+            --token ${{ secrets.GITHUB_TOKEN }} \
+            --issue-number $GITHUB_PR_NUMBER \
+            --start-rev $(git merge-base $START_REV $END_REV) \
+            --end-rev $END_REV \
+            --changed-files "$CHANGED_FILES"
+
+      - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 #v4.3.0
+        if: always()
+        with:
+          name: workflow-args
+          path: |
+            comments