Give microsoftDemangle() an outparam for how many input bytes were consumed.

Demangling Itanium symbols either consumes the whole input or fails,
but Microsoft symbols can be successfully demangled with just some
of the input.

Add an outparam that enables clients to know how much of the input was
consumed, and use this flag to give llvm-undname an opt-in warning
on partially consumed symbols.

Differential Revision: https://reviews.llvm.org/D80173