librustc: Limit the typo suggestions to reasonable suggests.

Impose a limit so that the typo suggester only shows reasonable
suggestions (i.e. don't suggest `args` when the error is `foobar`).
auto merge of #5579 : dbaupp/rust/rustc-typo-limit, r=catamorphism

Impose a limit so that the typo suggester only shows reasonable
suggestions (i.e. don't suggest `args` when the error is `foobar`).

A tiny bit of progress on #2281.