[test] Add a test for previously crashing interface gen. rdar://23544224

Author: nkcsgexi

test/SourceKit/InterfaceGen/Inputs/Foo3.swift

@@ -0,0 +1,8 @@
+public class C {
+  public func f1() {
+     { (error : NSError?) -> Void in
+      dispatch_async(dispatch_get_main_queue(), { () -> Void in
+      })
+    }
+  }
+}

test/SourceKit/InterfaceGen/gen_swift_source.swift

@@ -11,3 +11,6 @@
 
 // RUN: %sourcekitd-test -req=interface-gen %S/Inputs/UnresolvedExtension.swift -- %S/Inputs/UnresolvedExtension.swift | FileCheck -check-prefix=CHECK2 %s
 // CHECK2: extension ET
+
+// RUN: %sourcekitd-test -req=interface-gen %S/Inputs/Foo3.swift -- %S/Inputs/Foo3.swift | FileCheck -check-prefix=CHECK3 %s
+// CHECK3: public class C {