gmock-spec-builders: add support for non-moveable types.

Do this by ripping out the "untyped perform action" machinery, which isn't
necessary: we can simply template the entry point on the result type, and use
RAII to avoid the need to special case void. This makes it easier to understand
the code and harder to introduce type-related undefined behavior, to boot.
PiperOrigin-RevId: 451493451
Change-Id: I225305f83164752ca92f2916721972eafba33168

Author: jacobsa