[Demangle] refactor DLangDemangle to use std::string_view

Many existing methods of the D Language Demangler take a C style string
and return an adjusted pointer to the same object as the input string is
consumed.

Make it more obvious by changing the signatures to accept
std::string_view& when the input is modified vs a copy of a
std::string_view when the input is not.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D152177