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/Basic/SourceManager.h

@@ -201,7 +201,7 @@ public:
   }
 
   StringRef extractText(CharSourceRange Range,
-                        Optional<unsigned> BufferID = {}) const;
+                        Optional<unsigned> BufferID = Nothing) const;
 
   llvm::SMDiagnostic GetMessage(SourceLoc Loc, llvm::SourceMgr::DiagKind Kind,
                                 const Twine &Msg,