Sema: add support for `__attribute__((__swift_private__))`

This attribute allows declarations to be restricted to the framework
itself, enabling Swift to remove the declarations when importing
libraries.  This is useful in the case that the functions can be
implemented in a more natural way for Swift.

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

Differential Revision: https://reviews.llvm.org/D87720
Reviewed By: Aaron Ballman

clang/include/clang/Basic/Attr.td

@@ -2177,6 +2177,11 @@ def SwiftNewType : InheritableAttr {
   let HasCustomParsing = 1;
 }
 
+def SwiftPrivate : InheritableAttr {
+  let Spellings = [GNU<"swift_private">];
+  let Documentation = [SwiftPrivateDocs];
+}
+
 def NoDeref : TypeAttr {
   let Spellings = [Clang<"noderef">];
   let Documentation = [NoDerefDocs];

clang/include/clang/Basic/AttrDocs.td

@@ -3713,6 +3713,19 @@ ypedef.
   }];
 }
 
+def SwiftPrivateDocs : Documentation {
+  let Category = SwiftDocs;
+  let Heading = "swift_private";
+  let Content = [{
+Declarations marked with the ``swift_private`` attribute are hidden from the
+framework client but are still made available for use within the framework or
+Swift SDK overlay.
+
+The purpose of this attribute is to permit a more idomatic implementation of
+declarations in Swift while hiding the non-idiomatic one.
+  }];
+}
+
 def OMPDeclareSimdDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "#pragma omp declare simd";

clang/lib/Sema/SemaDeclAttr.cpp

@@ -7899,6 +7899,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_SwiftObjCMembers:
     handleSimpleAttribute<SwiftObjCMembersAttr>(S, D, AL);
     break;
+  case ParsedAttr::AT_SwiftPrivate:
+    handleSimpleAttribute<SwiftPrivateAttr>(S, D, AL);
+    break;
 
   // XRay attributes.
   case ParsedAttr::AT_XRayLogArgs:

clang/test/AST/attr-swift_private.m

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -ast-dump %s | FileCheck %s
+
+@interface I
+- (void)method __attribute__((__swift_private__));
+@end
+
+// CHECK: ObjCInterfaceDecl {{.*}} I
+// CHECK: ObjCMethodDecl {{.*}} method 'void'
+// CHECK: SwiftPrivateAttr
+
+@interface J : I
+- (void)method;
+@end
+
+// CHECK: ObjCInterfaceDecl {{.*}} J
+// CHECK: ObjCMethodDecl {{.*}} method 'void'
+// CHECK: SwiftPrivateAttr {{.*}} Inherited
+
+void f(void) __attribute__((__swift_private__));
+// CHECK: FunctionDecl {{.*}} f 'void (void)'
+// CHECK: SwiftPrivateAttr
+
+void f(void) {
+}
+// CHECK: FunctionDecl {{.*}} f 'void (void)'
+// CHECK: SwiftPrivateAttr {{.*}} Inherited

clang/test/SemaObjC/attr-swift_private.m

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -fobjc-arc %s
+
+__attribute__((__swift_private__))
+@protocol P
+@end
+
+__attribute__((__swift_private__))
+@interface I
+@end
+
+@interface J
+@property id property __attribute__((__swift_private__));
+- (void)instanceMethod __attribute__((__swift_private__));
++ (void)classMethod __attribute__((__swift_private__));
+@end
+
+void f(void) __attribute__((__swift_private__));
+
+struct __attribute__((__swift_private__)) S {};
+
+enum __attribute__((__swift_private__)) E {
+  one,
+  two,
+};
+
+typedef struct { } T __attribute__((__swift_private__));
+
+void g(void) __attribute__((__swift_private__("private")));
+// expected-error@-1 {{'__swift_private__' attribute takes no arguments}}