`--cap-lints allow` switches off `can_emit_warnings`

This boolean field on the error `Handler` is toggled to silence
warnings when `-A warnings` is passed. (This is actually a separate
mechanism from the global lint level—whether there's some redundancy
to be factored away here is an important question, but not one we
concern ourselves with in this commit.)  But the same rationale
applies for `--cap-lints allow`. In particular, this makes the "soft"
feature-gate warning introduced in 8492ad24 (which is not a lint, but
just calls `struct_span_warn`) not pollute the builds of dependent
crates.

Thanks to @kennytm for pointing out the potential of
`can_emit_warnings` for this purpose.

Resolves #44213.

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