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

[Sean Perry via cfe-commits]

https://github.com/perry-ca commented:

@AaronBallman Thanks for the feedback.  I'll make the changes and look into why _Exxport wasn't implemented like the other declspecs.

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.

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