[AST] In ObjCInterfaceDecl::isDesignatedInitializer(), use getMethod() instead of lookupMethod().

lookupMethod also goes through categories, which we don't need there.
[objcmt] Add a modernization option to infer and suggest designated initializers.

rdar://15509284
[Driver] Rename '-objcmt-white-list-dir-path' option to '-objcmt-whitelist-dir-path' and add an alias for now.