Omit @objcImpl from module interfaces

Module interfaces should not include the @objcImplementation attribute, member implementations that are redundant with the ObjC header, or anything that would be invalid in an ordinary extension (e.g. overridden initializers, stored Swift-only properties).

test/ModuleInterface/objc_implementation.swift

@@ -0,0 +1,84 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -I %S/Inputs/objc_implementation -F %clang-importer-sdk-path/frameworks %s -import-underlying-module -swift-version 5 -enable-library-evolution -emit-module-interface-path %t.swiftinterface
+// RUN: %FileCheck --input-file %t.swiftinterface %s
+// RUN: %FileCheck --input-file %t.swiftinterface --check-prefix NEGATIVE %s
+// REQUIRES: objc_interop
+
+// We should never see @_objcImplementation in the header
+// NEGATIVE-NOT: @_objcImplementation
+
+//
+// @_objcImplementation class
+//
+
+// CHECK-LABEL: extension objc_implementation.ImplClass {
+@_objcImplementation extension ImplClass {
+  // CHECK-NOT: init()
+  @objc public override init() {
+    implProperty = 42
+    implProperty2 = NSObject()
+    super.init()
+  }
+
+  // CHECK-NOT: var implProperty:
+  @objc public var implProperty: Int32 {
+    didSet { print(implProperty) }
+  }
+
+  // CHECK-DAG: final public var implProperty2: ObjectiveC.NSObject? { get set }
+  public final var implProperty2: NSObject?
+
+  // CHECK-NOT: func mainMethod
+  @objc public func mainMethod(_: Int32) { print(implProperty) }
+
+  // CHECK-NOT: deinit
+}
+// CHECK: }
+
+//
+// @_objcImplementation category
+//
+
+// Empty category should be omitted, so there's only one `extension ImplClass`.
+// CHECK-NOT: extension objc_implementation.ImplClass {
+@_objcImplementation(Category1) extension ImplClass {
+  // NEGATIVE-NOT: func category1Method
+  @objc public func category1Method(_: Int32) {
+    print("category1Method")
+  }
+}
+
+//
+// Second @_objcImplementation class, inherited initializer
+//
+
+// NEGATIVE-NOT: extension objc_implementation.NoInitImplClass
+@_objcImplementation extension NoInitImplClass {
+  // NEGATIVE-NOT: var s1:
+  @objc public let s1 = "s1v"
+  // NEGATIVE-NOT: var s2:
+  @objc public var s2 = "s2v"
+  // NEGATIVE-NOT: var s3:
+  @objc(s3) public let s3 = "s3v"
+  // NEGATIVE-NOT: var s4:
+  @objc(s4) public var s4 = "s4v"
+}
+
+//
+// @objc subclass of @_objcImplementation class
+//
+
+// CHECK-LABEL: @objc @_inheritsConvenienceInitializers open class SwiftSubclass : objc_implementation.ImplClass {
+open class SwiftSubclass: ImplClass {
+  // CHECK-DAG: @objc override dynamic open func mainMethod
+  override open func mainMethod(_: Int32) {
+    print("subclass mainMethod")
+  }
+
+  // CHECK-DAG: @objc dynamic public init()
+  // CHECK-DAG: @objc deinit
+}
+// CHECK: }
+
+//
+// Epilogue
+//