All except
`toolchain/check/testdata/interop/cpp/function_param_int*.carbon` and
`toolchain/check/testdata/interop/cpp/function_return.carbon`.
Don't output SemIR for cases that are intended to fail.
This mirrors how imports work in general, that the imported declarations
shouldn't belong to the first referencing scope (particularly apparent
when referenced across multiple scopes). I think this was just an
oversight here.
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.