Add span_lint_note to the lint Context

[kmcallister]

This calls span_note unless the lint level is Allow, in which case it does nothing.

[andrewyli]

Why isn't span_lint in Context good enough?

[kmcallister]

Because it prints a warning or an error, not a note.

[andrewyli]

It seems to me that span_lint_note would mostly be comprised of code similar to/from raw_emit_lint, with only minor changes. Is that redundant or am I missing something?
Thanks in advance.

[huonw]

Theoretically we may want span_lint_help too.

[andrewyli]

For all these versions of what currently exists, probably need a modification of raw_emit_lint to deal with all cases like span_lint_x.
(I'll take a stab if that's okay)

[kmcallister]

It'd be great to combine them, yeah.

[nxnfufunezn]

working on this..