CI workflow to check clang-format usage on pull requests

Adapted from LLVM version of pr-code-format.yml.
Copies a few scripts from LLVM to External/.
Runs self-hosted on X64.

Assumes clang-format 16.0.6 for formatting.

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

@@ -0,0 +1,75 @@
+# 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: [self-hosted, X64]
+    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
+
+      - name: "Listed files"
+        env:
+          CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
+        run: |
+          echo "Formatting files:"
+          echo "$CHANGED_FILES"
+
+      - name: Check for correct clang-format version
+        run: clang-format --version | grep -qF '16.0.6'
+
+      - name: Check git-clang-format-16 exists
+        run: which git-clang-format-16
+
+      - name: Setup Python env
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.11'
+          cache: 'pip'
+          cache-dependency-path: './External/code-format-helper/requirements_formatting.txt'
+
+      - name: Install python dependencies
+        run: pip install -r ./External/code-format-helper/requirements_formatting.txt
+
+      - name: Run code formatter
+        env:
+          CLANG_FORMAT_PATH: 'git-clang-format-16'
+          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(pmatos): Once we adopt v18, 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.
+        run: |
+          python ./External/code-format-helper/code-format-helper.py \
+            --repo "FEX-emu/FEX" \
+            --issue-number $GITHUB_PR_NUMBER \
+            --start-rev $(git merge-base $START_REV $END_REV) \
+            --end-rev $END_REV \
+            --changed-files "$CHANGED_FILES"