[Backtracing][Linux] Add a comment about __cxa_demangle.
`__cxa_demangle()` is a rather unusual API; one of its "features" is that
the pointer you pass in must either be `nullptr`, in which case it
will call `malloc()` itself, _or_ it has to be a pointer to a block
of memory allocated with `malloc()`, because `__cxa_demangle()` may
`realloc()` it for you.
This seems to me to be something of a non-obvious footgun, so we never
pass the caller's pointer through to `__cxa_demangle()`, which lets them
decide how they want to allocate space.
rdar://110261712
