fix warnings in Demangle.cpp
fix warnings in Demangle.cpp

Summary: Fix unused-argument warnings.

Reviewed By: yfeldblum

Differential Revision: D15068741

fbshipit-source-id: 486451bd9bd8575bed8a647f4531783c82c079b5

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly