stdlib: rename Reflectable.{getMirror() -> _getMirror()}

Start removing the old mirrors.

rdar://21428474

Swift SVN r29823

stdlib/public/core/StringUnicodeScalarView.swift

@@ -206,7 +206,7 @@ extension String {
     }
 
     /// Returns a mirror that reflects `self`.
-    public func getMirror() -> MirrorType {
+    public func _getMirror() -> MirrorType {
       return _UnicodeScalarViewMirror(self)
     }