This drops file_test runtime from about 2.5s to 2s on my machine, which
is now ~50% faster than before
https://github.com/carbon-language/carbon-lang/pull/5653 slowed things
down by adding a lot of stuff to the production prelude.
People seemed receptive [on
#toolchain](https://discord.com/channels/655572317891461132/655578254970716160/1379196447827689553),
so proceeding.
For the two name conflicts, I've set it up so that there's a
"foo.carbon" and "foo_with_prelude.carbon", to indicate that the
no-prelude approach is preferred (with a shorter name).
Note min_prelude will require a little more work/thought, I want to
avoid adding `--custom-core` etc.
Dana Jansens
Jon Ross-Perkins