[demangler][NFC] Rename SwapAndRestore to ScopedOverride

The demangler has a utility class 'SwapAndRestore'. That name is
confusing. It's not swapping anything, and the restore part happens at
the object's destruction. What it's actually doing is allowing a
override of some value that is dynamically accessible within the
lifetime of a lexical scope. Thus rename it to ScopedOverride, and
tweak it's member variable names.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D122606