[Sema] Always accept access-level on imports in swiftinterface

While use of access-level in imports is still gated by a flag for normal
source code, accept it in swiftinterfaces without the flag. This will
make it easier for older compilers to read newer swiftinterfaces created
by a future compiler where that restriction is lifted.

rdar://115455383

Author: xymus

lib/Sema/TypeCheckAttr.cpp

@@ -968,7 +968,8 @@ void AttributeChecker::visitLazyAttr(LazyAttr *attr) {
 
 bool AttributeChecker::visitAbstractAccessControlAttr(
     AbstractAccessControlAttr *attr) {
-  // Access control attr may only be used on value decls and extensions.
+  // Access control attr may only be used on value decls, extensions and
+  // imports.
   if (!isa<ValueDecl>(D) && !isa<ExtensionDecl>(D) && !isa<ImportDecl>(D)) {
     diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr);
     return true;
@@ -996,7 +997,9 @@ bool AttributeChecker::visitAbstractAccessControlAttr(
   }
 
   if (auto importDecl = dyn_cast<ImportDecl>(D)) {
-    if (!D->getASTContext().LangOpts.hasFeature(Feature::AccessLevelOnImport)) {
+    SourceFile *File = D->getDeclContext()->getParentSourceFile();
+    if (!D->getASTContext().LangOpts.hasFeature(Feature::AccessLevelOnImport) &&
+        File && File->Kind != SourceFileKind::Interface) {
       diagnoseAndRemoveAttr(attr, diag::access_level_on_import_not_enabled);
       return true;
     }

test/Sema/access-level-import-flag-check.swift

@@ -16,6 +16,9 @@
 // RUN:   -package-name package
 // REQUIRES: asserts
 
+/// swiftinterfaces don't need the flag.
+// RUN: %target-swift-typecheck-module-from-interface(%t/Client.swiftinterface)
+
 //--- PublicLib.swift
 //--- PackageLib.swift
 //--- InternalLib.swift
@@ -28,3 +31,8 @@ package import PackageLib // expected-error@:1 {{Access level on imports require
 internal import InternalLib // expected-error@:1 {{Access level on imports require '-enable-experimental-feature AccessLevelOnImport'}}
 fileprivate import FileprivateLib // expected-error@:1 {{Access level on imports require '-enable-experimental-feature AccessLevelOnImport'}}
 private import PrivateLib // expected-error@:1 {{Access level on imports require '-enable-experimental-feature AccessLevelOnImport'}}
+
+//--- Client.swiftinterface
+// swift-interface-format-version: 1.0
+// swift-module-flags: -enable-library-evolution
+public import Swift