[Damangle] convert rustDemangle to use std::string_view

I was doing this API conversion to use std::string_view top-down in
D149104, but this exposed issues in individual demanglers that needed to
get fixed first. There's no issue with the conversion for the Rust
demangler, so convert it first.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149784