Sema: Fix bug with private access scopes in Swift 3 mode

In Swift 3 mode, the canonical private DeclContext should
not look through extensions.

The only way I can reproduce this is with a missing warning
and we don't really care about missing warnings in Swift 3 mode.

However, the next patch in this PR regresses more things so
let's fix it properly.

Author: slavapestov

lib/AST/DeclContext.cpp

@@ -821,6 +821,9 @@ IterableDeclContext::castDeclToIterableDeclContext(const Decl *D) {
 /// declaration or extension, the supplied context is returned.
 static const DeclContext *
 getPrivateDeclContext(const DeclContext *DC, const SourceFile *useSF) {
+  if (DC->getASTContext().isSwiftVersion3())
+    return DC;
+
   auto NTD = DC->getAsNominalTypeOrNominalTypeExtensionContext();
   if (!NTD)
     return DC;

test/Compatibility/accessibility_private.swift

@@ -171,8 +171,10 @@ extension Container {
 extension Container {
   private struct VeryPrivateStruct { // expected-note * {{type declared here}}
     private typealias VeryPrivateType = Int // expected-note * {{type declared here}}
+    private struct VeryPrivateInnerStruct {}
     var privateVar: VeryPrivateType { fatalError() } // expected-warning {{property should be declared private because its type uses a private type}}
     var privateVar2 = VeryPrivateType() // expected-warning {{property should be declared private because its type 'Container.VeryPrivateStruct.VeryPrivateType' (aka 'Int') uses a private type}}
+    var privateVar3 = VeryPrivateInnerStruct() // expected-warning {{property should be declared private because its type 'Container.VeryPrivateStruct.VeryPrivateInnerStruct' uses a private type}}
     typealias PrivateAlias = VeryPrivateType // expected-warning {{type alias should be declared private because its underlying type uses a private type}}
     subscript(_: VeryPrivateType) -> Void { return () } // expected-warning {{subscript should be declared private because its index uses a private type}}
     func privateMethod(_: VeryPrivateType) -> Void {} // expected-warning {{method should be declared private because its parameter uses a private type}} {{none}}

test/Sema/accessibility_private.swift

@@ -171,8 +171,10 @@ extension Container {
 extension Container {
   private struct VeryPrivateStruct { // expected-note * {{type declared here}}
     private typealias VeryPrivateType = Int // expected-note * {{type declared here}}
+    private struct VeryPrivateInnerStruct {}
     var privateVar: VeryPrivateType { fatalError() } // expected-error {{property must be declared private because its type uses a private type}}
     var privateVar2 = VeryPrivateType() // expected-error {{property must be declared private because its type 'Container.VeryPrivateStruct.VeryPrivateType' (aka 'Int') uses a private type}}
+    var privateVar3 = VeryPrivateInnerStruct() // expected-error {{property must be declared private because its type 'Container.VeryPrivateStruct.VeryPrivateInnerStruct' uses a private type}}
     typealias PrivateAlias = VeryPrivateType // expected-error {{type alias must be declared private because its underlying type uses a private type}}
     subscript(_: VeryPrivateType) -> Void { return () } // expected-error {{subscript must be declared private because its index uses a private type}}
     func privateMethod(_: VeryPrivateType) -> Void {} // expected-error {{method must be declared private because its parameter uses a private type}} {{none}}