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

[Sean Perry via cfe-commits]

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perry-ca wrote:

I'd like to get this as simple as possible.  This is very similar to `pragma weak`.  The addition of the pragma is to help with porting code over to clang rather than writing new code.  We would like to encourage people to use `__attribuite__((visibility("default"))` instead of the pragma in C++ programs.  Adding support for fully qualified names opens the door to supporting dependent names, overloads, etc.  This pragma was only ever used by C programs so limiting it to simple names doesn't impose any restrictions on ported code.

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