[AST] Don't print using declarations in swift interfaces

These declarations are effectively `fileprivate` at the moment
and should appear in swift interfaces.

lib/AST/ASTPrinter.cpp

@@ -383,6 +383,11 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(ModuleDecl *ModuleToPrint,
         }
       }
 
+      // The `using` declarations are private to the file at the moment
+      // and shouldn't appear in swift interfaces.
+      if (isa<UsingDecl>(D))
+        return false;
+
       return ShouldPrintChecker::shouldPrint(D, options);
     }
   };

test/ModuleInterface/using.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s
+
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface)
+
+// RUN: %FileCheck %s --input-file %t.swiftinterface
+
+using @MainActor
+
+// CHECK-NOT: using @MainActor