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

[Sean Perry via cfe-commits]

perry-ca wrote:

>It looks like you're NOT supporting external functions, every example I see is a locally defined function?
I take by this you mean the function needs to be defined in the same compilation unit as the pragma appears.  That is correct.  The visibility is attached to the function definition in the object file.  The documentation states this requirement.

I have created the documentation for the pragma in the language extensions file.  I'll be happy to revise/expand that.

I have also addressed the dependent name question.  The pragma can't be used within a struct definition so that example produces an error.  The pragma has to be used at file scope.  That is stated in the docs.  I was allowing the pragma inside of function bodies.  I removed that in the latest commit too so everything is consistent.

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