[Bug c/57612] add builtin to assert that expression does not have side effects

[tromey at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57612

--- Comment #1 from Tom Tromey <tromey at gcc dot gnu.org> ---
You can see the thread here:

http://patchwork.ozlabs.org/patch/343858/

My proposed patch didn't handle C++, which seems like
something it probably should do.