[clang] [SystemZ][z/OS] Add visibility features for z/OS (eg. _Export, pragma export) (PR #111035)

[Erich Keane via cfe-commits]

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@@ -7552,6 +7552,27 @@ attribute requires a string literal argument to identify the handle being releas
   }];
 }
 
+def zOSExportDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Use the ``_Export`` keyword on a function or external variable to declare
+that it is to be exported (made available to other modules). You must define
+the object name in the same translation unit in which you use the ``_Export``
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erichkeane wrote:

This documentation should also mention the pragma spelling and document the whole thing.  Also, do we diagnose that `anothony` isn't defined?

https://github.com/llvm/llvm-project/pull/111035