Don't synthesize initializers in swiftinterface files

...and then don't complain about a class not having any initializers.

lib/Parse/ParseDecl.cpp

@@ -5481,6 +5481,8 @@ ParserResult<EnumDecl> Parser::parseDeclEnum(ParseDeclOptions Flags,
                                         { }, nullptr, CurDeclContext);
   setLocalDiscriminator(ED);
   ED->getAttrs() = Attributes;
+  if (SF.Kind == SourceFileKind::Interface)
+    ED->setAddedImplicitInitializers();
 
   ContextChange CC(*this, ED);
 
@@ -5753,6 +5755,8 @@ ParserResult<StructDecl> Parser::parseDeclStruct(ParseDeclOptions Flags,
                                             CurDeclContext);
   setLocalDiscriminator(SD);
   SD->getAttrs() = Attributes;
+  if (SF.Kind == SourceFileKind::Interface)
+    SD->setAddedImplicitInitializers();
 
   ContextChange CC(*this, SD);
 
@@ -5840,9 +5844,9 @@ ParserResult<ClassDecl> Parser::parseDeclClass(ParseDeclOptions Flags,
   ClassDecl *CD = new (Context) ClassDecl(ClassLoc, ClassName, ClassNameLoc,
                                           { }, nullptr, CurDeclContext);
   setLocalDiscriminator(CD);
-
-  // Attach attributes.
   CD->getAttrs() = Attributes;
+  if (SF.Kind == SourceFileKind::Interface)
+    CD->setAddedImplicitInitializers();
 
   ContextChange CC(*this, CD);
 

lib/Sema/TypeCheckDecl.cpp

@@ -5138,6 +5138,19 @@ static void diagnoseClassWithoutInitializers(TypeChecker &tc,
 }
 
 void TypeChecker::maybeDiagnoseClassWithoutInitializers(ClassDecl *classDecl) {
+  if (auto *SF = classDecl->getParentSourceFile()) {
+    // Allow classes without initializers in SIL and textual interface files.
+    switch (SF->Kind) {
+    case SourceFileKind::SIL:
+    case SourceFileKind::Interface:
+      return;
+    case SourceFileKind::Library:
+    case SourceFileKind::Main:
+    case SourceFileKind::REPL:
+      break;
+    }
+  }
+
   // Some heuristics to skip emitting a diagnostic if the class is already
   // irreperably busted.
   if (classDecl->isInvalid() ||

test/ModuleInterface/SmokeTest.swiftinterface

@@ -29,6 +29,14 @@ public class TestClass {
   deinit
 } // CHECK: {{^}$}}
 
+// CHECK-LABEL: public class TestEmptyClass {
+public class TestEmptyClass {
+} // CHECK-NEXT: {{^}$}}
+
+// CHECK-LABEL: public struct TestEmptyStruct {
+public struct TestEmptyStruct {
+} // CHECK-NEXT: {{^}$}}
+
 // CHECK-LABEL: public enum TestEnum
 public enum TestEnum {
   // CHECK: case a