Use Nothing instead of {} to initialize Optionals.

In preparation for the switch to llvm::Optional, which has an explicit
default constructor.

Swift SVN r22470

include/swift/Serialization/ModuleFile.h

@@ -520,7 +520,7 @@ public:
   ///                      kinds of decls, used to avoid re-entrant
   ///                      deserialization.
   Decl *getDecl(serialization::DeclID DID,
-                Optional<DeclContext *> ForcedContext = {});
+                Optional<DeclContext *> ForcedContext = Nothing);
 
   /// Returns the decl context with the given ID, deserializing it if needed.
   DeclContext *getDeclContext(serialization::DeclID DID);