[Damangle] convert dlangDemangle 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 D language
demangler, so convert it.

I have a more aggressive refactoring of the entire D language demangler
to use std::string_view more extensively, but the interface with
llvm::nonMicrosoftDemangle is the more interesting one.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D151003