[clang] [SystemZ][z/OS] Implement #pragma export (PR #141671)

[Sean Perry via cfe-commits]

perry-ca wrote:

> I mentioned above, its missing a massive amount of dependent cases/etc. Also, the not-allowing-FQNs seems unfortunate, though you're at least testing those as errors.
> 
The pragma only supports external function and variable names and doesn't support fully qualified names.  I don't think you can have a valid case where the name is a dependent name.  Do you have an example of what you are thinking of?

> As far as documentation, we have LanguageExtensions.rst that I think this fits into.
I've got that written and will add the commit shortly.


https://github.com/llvm/llvm-project/pull/141671