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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I did consider these to be the same feature (two ways to set the same information). If it's easier for reviewing I can look into splitting the PR.

I don't insist on a split, FWIW.

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