[Serialization] Prevent using declarations from being serialized

(cherry picked from commit bc61bfb)

Author: xedin

lib/Serialization/Serialization.cpp

@@ -6968,7 +6968,7 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
 
     for (auto D : fileDecls) {
       if (isa<ImportDecl>(D) || isa<MacroExpansionDecl>(D) ||
-          isa<TopLevelCodeDecl>(D)) {
+          isa<TopLevelCodeDecl>(D) || isa<UsingDecl>(D)) {
         continue;
       }
 

test/Serialization/using.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t %s -module-name Using -enable-experimental-feature DefaultIsolationPerFile
+// RUN: %target-swift-frontend -typecheck -I %t %s -module-name main -DMAIN -verify -enable-experimental-feature DefaultIsolationPerFile
+
+// REQUIRES: swift_feature_DefaultIsolationPerFile
+
+using @MainActor
+
+public func test() {}