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

[Sean Perry via cfe-commits]

perry-ca wrote:

> I only made it through the first bit of hte implementation, but parsing doesn't seem right to me, it doesn't recovery nicely as far as I can tell.
> 
> AS far as the design here, it isn't clear to me how this manages all of the C++ lookup/scoping/etc, and I don't see how the data structure provided would fix that.
> 
> Also, you seem to be doing some sort of parse during statement parsing, but no tests that show what this does/how this works when the scope isn't the local one? Or when this is in a function definition/etc.
> 
> ALSO: I don't see any template instantiation code, which this would need for statement/decl level declaration.

This pragma isn't meant to be able to accept all valid C++ declarations.  It only accept two forms in the XL C/C++ compiler.  These are:
- just an identifier.  This will match a variable or an function declared as extern "C" (eg. `foo`)
- an identifier + an argument list.  This will match a basic function declarator.  The intent is to accept something like `sin(double)` and no more.  This is to match the XL C/C++ behaviour.  The system header have pragmas in this form.  

I am going to remove the code to handle the second form from this PR and focus on providing just the basic identifier form.  I'll update the description to reflect this.


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