Lint if args adapted to Unit and discarded

[som-snytt]

Fixes scala/bug#12495

[SethTisue]

Is there an existing flag this could or should be folded into? (As you may recall, I had the same reaction to -Wnonunit-statement and -Wvalue-discard and you convinced me it made sense for them to be separate. But now this goes to three.)

[som-snytt]

-Wvalue-discard is not a lint because it is a commonly-leveraged language feature.

This lint is stronger than the adapted-args lint because merely adapted implies still consumed and not discarded.

This is a lint because throwing away method args is not a commonly-leveraged language feature. To quote the paulpism, "this is unlikely to be what you want."

I see from the test that -Wvalue-discard still correctly warns. I think that is fine. The clever messaging: "args will be discarded" and "value was discarded."

[lrytz]

looks good to me, @SethTisue ?