[demangler] Add StringView conversion operator

The OutputBuffer class tries to present a NUL-terminated string API to
consumers.  But several of them would prefer a StringView.  In
particular the Microsoft demangler, juggles between NUL-terminated and
StringView, which is confusing.

This adds a StringView conversion, and adjusts the Demanglers that can
benefit from that.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D120990