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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

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.

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