Auto merge of #42850 - estebank:unwanted-return-rotj, r=nikomatsakis

Auto merge of #42850 - estebank:unwanted-return-rotj, r=nikomatsakis

Detect missing `;` on methods with return type `()`

 - Point out the origin of a type requirement when it is the return type
   of a method
 - Point out possibly missing semicolon when the return type is `()` and
   the implicit return makes sense as a statement
 - Suggest changing the return type of methods with default return type
 - Don't suggest changing the return type on `fn main()`
 - Don't suggest changing the return type on impl fn
 - Suggest removal of semicolon (instead of being help)