[API Notes] Test SwiftPrivate API notes.

*Clang update required*

Test that rdar://problem/25872038 works for Swift.

Author: DougGregor

test/APINotes/Inputs/custom-modules/APINotesTest.apinotes

@@ -5,6 +5,8 @@ Functions:
 Globals:
   - Name: ANTGlobalValue
     SwiftName: global
+  - Name: will_be_private
+    SwiftPrivate: true
 Tags:
   - Name: PointStruct
     SwiftName: Point

test/APINotes/Inputs/custom-modules/APINotesTest.h

@@ -10,3 +10,5 @@ typedef double real_t;
 typedef struct {
   struct PointStruct topLeft, bottomRight;
 } RectStruct;
+
+extern double will_be_private;

test/APINotes/basic.swift

@@ -15,4 +15,6 @@ func testSwiftName() {
 
   let rect: Rect
   let rect2: RectStruct // expected-error{{use of undeclared type 'RectStruct'}}
+
+  let d: Double = __will_be_private
 }