This is a workaround for InternPool currently not handling
non-null-terminated strings. It avoids using the `bytes` storage for
aggregates if there are any null bytes.

In the future this should be changed so that the `bytes` storage can be
used regardless of whether there are any null bytes. This is important
for use cases such as `@embedFile`.

However, this fixes a bug for now, and after this commit, stage2
self-hosts again.

mlugg: stage5 passes all enabled behavior tests on my system.

Commit message edited by Andrew Kelley <andrew@ziglang.org>