[demangler][NFC] Utility header cleanups

a) Using a do...while loop in the number formatter means we do not
have to special case zero.

b) Let's use 'if (auto size = ...) {}' for appending to the output
buffer.

c) We should also be using memcpy there, not memmove -- the string
being appended is never part of the current buffer.

d) Let's put all the operator<< functions together.

e) I find 'if (cond) frob(..., true) ; elseOD frob(..., false)'
somewhat confusing.  Let's just use std::abs in the signed integer
printer and let CSE decide about the duplicate < 0 testing.

f) Let's have as many as possible return *this.  That's both more
consistent, and allows tailcalls in some cases (the actual number
formatter has a local array though).

These changes removed around 100 bytes from the demangler's
instructions on x86_64.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119176