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

[Sean Perry via cfe-commits]

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@@ -5087,6 +5087,19 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
   assert(EllipsisLoc.isInvalid() &&
          "Friend ellipsis but not friend-specified?");
 
+  if (DS.isExportSpecified()) {
+    VisibilityAttr *existingAttr = TagD->getAttr<VisibilityAttr>();
+    if (existingAttr) {
+      VisibilityAttr::VisibilityType existingValue =
+          existingAttr->getVisibility();
+      if (existingValue != VisibilityAttr::Default)
+        Diag(DS.getExportSpecLoc(), diag::err_mismatched_visibility);
+    } else {
+      Tag->addAttr(
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perry-ca wrote:

Totally valid.  The intention was to have these be another way to specify `__attribute__((visibility("default")))`.  I've rewritten the code to these issues and have hopefully addressed all of your concerns.  

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