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

[Louis Dionne via llvm-commits]

ldionne wrote:

> Can we just use & fix the existing formatting check?

As I explained on Discord:

> There are two CI jobs in question here:
>
> 1. The formatting job that was set up by @tobiashieta . This job ensures that whatever changes have been touched in a PR are formatted properly except for any file in ignore_format.txt. That job works as intended and the only problem with it is that it skips libc++ headers that have no extension.
> 2. The check-generated-output job. That job runs various generation scripts to re-generate our modulemaps and a few other things, and then ensures that they have been re-generated by whoever created a PR. As part of this job, we ensure that the list of files to ignore format in (ignore_format.txt) is kept up to date. 90% of the problems we've been talking about in this thread have to do with the fact that this job has a few flaws

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