[demangler] Simplify OutputBuffer initialization

Every non-testcase use of OutputBuffer contains code to allocate an
initial buffer (using either 128 or 1024 as initial guesses). There's
now no need to do that, given recent changes to the buffer extension
heuristics -- it allocates a 1k(ish) buffer on first need.

Just pass in a buffer (if any) to the constructor.  Thus the
OutputBuffer's ownership of the buffer starts at its own lifetime
start. We can reduce the lifetime of this object in several cases.

That new constructor takes a 'size_t *' for the size argument, as all
uses with a non-null buffer are passing through a malloc'd buffer from
their own caller in this manner.

The buffer reset member function is never used, and is deleted.

Some adjustment to a couple of uses is needed, due to the lazy buffer
creation of this patch.

a) the Microsoft demangler can demangle empty strings to nothing,
which it then memoizes.  We need to avoid the UB of passing nullptr to
memcpy.

b) a unit test checks insert...