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

[Louis Dionne via llvm-commits]

ldionne wrote:

Sorry for the churn on the patch -- AFAICT this should be ready to review again now. I'm using `on: pull_request` instead to avoid security issues, and I tested this in https://github.com/ldionne/llvm-project/pull/4.

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