[llvm] [libc++] Move the check-generated-files job to Github Actions (PR #68920)

[Tom Stellard via llvm-commits]

================
@@ -0,0 +1,29 @@
+name: "Check libc++ generated files"
+on:
+  pull_request_target:
+    paths:
+      - 'libcxx/**'
+permissions:
+  pull-requests: write
+
+jobs:
+  check_generated_files:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fetch LLVM sources
+        uses: actions/checkout at v4
+        with:
+          fetch-depth: 2
+
+      - name: Install clang-format
+        uses: aminya/setup-cpp at v1
+        with:
+          clangformat: 17.0.1
+
+      - name: Install Ninja
+        uses: seanmiddleditch/gha-setup-ninja at master
----------------
tstellar wrote:

We have our own action for installing ninja:  llvm/actions/install-ninja

But do we really need this if the job is always going to run on Linux?  Can we just apt install it?

https://github.com/llvm/llvm-project/pull/68920