bug#34789: mantemp.el should be obsoleted

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Fri, 8 Mar 2019 11:16:27 -0800
> 
> Davis Herring writes, "One of your fixes was to progmodes/mantemp.el,
> which (now that I know it exists) I'm pretty sure should be obsoleted;
> as you may already know, C++ compilers haven't worked that way for any
> standard library type (and most user-defined types) in a very long time."
> 
> I notice that no significant changes have been made to mantemp.el since
> 1997. As it's evidently useless nowadays, I propose that we move
> mantemp.el to the obsolete area, and remove it in a later Emacs release.

If no compiler can ever be helped by that feature, I think I agree.
But I'd like first to hear in more detail why "C++ compilers haven't
worked that way for any standard library type (and most user-defined
types) in a very long time."  We should at least have this information
recorded here for posterity.

Thanks.