Show lint names

[sammykim]

Fix #3525.

[sammykim]

Done.

[alexcrichton]

When merging this with #6093, I went ahead and made these a bit more specific. It's not always the case that -W should be in front from what I could tell. There's three different cases of source emission now:

• The lint level was by default, and the message has [(warn|forbid|deny) by default]
• The lint level was set by the command line, and the appended message is [-(W|D|F) lint-name]
• The lint level was set by some attribute, in which case a span_note is emitted with the source of the lint source.

Does that sound ok to you?

[sanxiyn]

@alexcrichton The idea of #3525 was that the lint should always show its name so that its setting can be changed. [warn by default] would not accomplish that.

[alexcrichton]

hmm... good point, I think I'll change it to [-W unused-imports (default)] just so it's known that it's a default instead of being passed on the command line.