[demangler] Make OutputBuffer non-copyable

In addressing the buffer ownership API, I discovered a rogue member
function that returned by value rather than by reference. It clearly
intended to return by reference, but because the copy ctor wasn't
deleted this wasn't caught.

It is not necessary to make this a move-only type, although that would
be an alternative.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D120901