[clang] [SystemZ][z/OS] Add visibility features for z/OS (eg. _Export, pragma export) (PR #111035)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

Thanks for this! Some things that are missing:

* a release note in `clang/docs/ReleaseNotes.rst`
* Parsing and Sema tests

Also, it looks like these are two separate (but related) features, one for the pragma and another for `_Export`. Is that the case? If so, they should be split into separate PRs.

https://github.com/llvm/llvm-project/pull/111035