Change default lint output format.

Since lint check attributes are the preferred way of selectively
enabling/disabling lint checks, the output format of a failed
default check has been changed to reflect that.

When lint checks are being explicitly set by a command-line flag
or an attribute, the behavior is unchanged, so that the user can
quickly pinpoint the source.

Closes #6580
auto merge of #9782 : geoffhill/rust/lint-format2, r=alexcrichton

Since lint check attributes are the preferred way of selectively
enabling/disabling lint checks, the output format of a failed
default check has been changed to reflect that.

When lint checks are being explicitly set by a command-line flag
or an attribute, the behavior is unchanged, so that the user can
quickly pinpoint the source.

Supersedes the patch suggested in #9778
Closes #6580