[demangler] Simplify SwapAndRestore

The SwapAndRestore class is over engineered.  Nothing makes use of the
early restoration machinery.  Let's just remove that cognative burdon.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D120673

GitOrigin-RevId: e5c98e22fbbe3c3eb5f2c782d0789098afc81518