Sema: add support for __attribute__((__swift_objc_members__))

This adds the `__swift_objc_members__` attribute to the semantic
analysis.  It allows for annotating ObjC interfaces to provide Swift
semantics indicating that the types derived from this interface will be
back-bridged to Objective-C to allow interoperability with Objective-C
and Swift.

This is based on the work of the original changes in
8afaf3aad2

Differential Revision: https://reviews.llvm.org/D87395
Reviewed By: Aaron Ballman, Dmitri Gribenko

clang/include/clang/Basic/Attr.td

@@ -2130,6 +2130,12 @@ def Regparm : TypeAttr {
   let ASTNode = 0;
 }
 
+def SwiftObjCMembers : Attr {
+  let Spellings = [GNU<"swift_objc_members">];
+  let Subjects = SubjectList<[ObjCInterface], ErrorDiag>;
+  let Documentation = [SwiftObjCMembersDocs];
+}
+
 def SwiftError : InheritableAttr {
   let Spellings = [GNU<"swift_error">];
   let Args = [

clang/include/clang/Basic/AttrDocs.td

@@ -3476,6 +3476,16 @@ Swift.
   }];
 }
 
+def SwiftObjCMembersDocs : Documentation {
+  let Category = SwiftDocs;
+  let Heading = "swift_objc_members";
+  let Content = [{
+This attribute indicates that Swift subclasses and members of Swift extensions
+of this class will be implicitly marked with the ``@objcMembers`` Swift
+attribute, exposing them back to Objective-C.
+  }];
+}
+
 def SwiftErrorDocs : Documentation {
   let Category = SwiftDocs;
   let Heading = "swift_error";

clang/lib/Sema/SemaDeclAttr.cpp

@@ -7536,6 +7536,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_SwiftError:
     handleSwiftError(S, D, AL);
     break;
+  case ParsedAttr::AT_SwiftObjCMembers:
+    handleSimpleAttribute<SwiftObjCMembersAttr>(S, D, AL);
+    break;
 
   // XRay attributes.
   case ParsedAttr::AT_XRayLogArgs:

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -150,6 +150,7 @@
 // CHECK-NEXT: SwiftError (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: SwiftErrorResult (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: SwiftIndirectResult (SubjectMatchRule_variable_is_parameter)
+// CHECK-NEXT: SwiftObjCMembers (SubjectMatchRule_objc_interface)
 // CHECK-NEXT: TLSModel (SubjectMatchRule_variable_is_thread_local)
 // CHECK-NEXT: Target (SubjectMatchRule_function)
 // CHECK-NEXT: TestTypestate (SubjectMatchRule_function_is_member)

clang/test/SemaObjC/attr-swift_objc_members.m

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+
+#if !__has_attribute(swift_objc_members)
+#error cannot verify presence of swift_objc_members attribute
+#endif
+
+__attribute__((__swift_objc_members__))
+__attribute__((__objc_root_class__))
+@interface I
+@end
+
+__attribute__((swift_objc_members))
+@protocol P
+@end
+// expected-error@-3 {{'swift_objc_members' attribute only applies to Objective-C interfaces}}
+
+__attribute__((swift_objc_members))
+extern void f(void);
+// expected-error@-2 {{'swift_objc_members' attribute only applies to Objective-C interfaces}}
+
+// expected-error@+1 {{'__swift_objc_members__' attribute takes no arguments}}
+__attribute__((__swift_objc_members__("J")))
+@interface J
+@end