DiagnosticEngine: Do not describe an extension's nominal for %kindonly

This is the desired behavior is most cases. In the future, we should
consider adding format specifiers for short/detailed descriptions.

Author: AnthonyLatsis

lib/AST/DiagnosticEngine.cpp

@@ -875,21 +875,23 @@ static void formatDiagnosticArgument(StringRef Modifier,
     }
 
     if (includeName) {
-      // If it's an accessor, describe that and then switch to discussing its
-      // storage.
       if (auto accessor = dyn_cast<AccessorDecl>(D)) {
+        // If it's an accessor, describe that and then switch to discussing its
+        // storage declaration.
         Out << Decl::getDescriptiveKindName(D->getDescriptiveKind()) << " for ";
         D = accessor->getStorage();
+      } else if (auto ext = dyn_cast<ExtensionDecl>(D)) {
+        // If it's an extension with a valid bound type declaration, describe
+        // the extension itself then switch to discussing the bound
+        // declaration.
+        if (auto *nominal = ext->getSelfNominalTypeDecl()) {
+          Out << Decl::getDescriptiveKindName(D->getDescriptiveKind())
+              << " of ";
+          D = nominal;
+        }
       }
     }
 
-    // If it's an extension, describe that and then switch to discussing its
-    // nominal type.
-    if (auto ext = dyn_cast<ExtensionDecl>(D)) {
-      Out << Decl::getDescriptiveKindName(D->getDescriptiveKind()) << " of ";
-      D = ext->getSelfNominalTypeDecl();
-    }
-
     // Figure out the name we want to print.
     DeclName name;
     if (includeName) {