[llvm] [Github][RFC] Add workflow to diff codegen on llvm-test-suite (PR #190010)

[Aiden Grossman via llvm-commits]

================
@@ -0,0 +1,136 @@
+# When /test-suite is commented on a PR, checks out the PR, builds clang and
+# then the test-suite in several configurations. It then checks out the base of
+# the PR, builds clang and the test-suite again, and then uploads the diff of
+# the codegen.
+
+name: Diff test-suite codegen
+
----------------
boomanaiden154 wrote:

Needs a top level
```yaml
permissions:
  contents: read
```

https://llvm.org/docs/CIBestPractices.html#top-level-read-permissions

https://github.com/llvm/llvm-project/pull/190010