Provide a clearer error message for semicolons after structs and similar

[joshtriplett]

Right now, writing something like struct S {}; just produces a parse error:

error: expected item, found `;`
 --> src/main.rs:1:12
  |
1 | struct S {};
  |            ^ help: consider removing this semicolon

We could make this error message much clearer by recognizing this common error and specifically saying "don't put a semicolon after a struct/union/enum declaration".

[PramodBisht]

@joshtriplett just to clarify should we change help or error message here?

[PramodBisht]

@joshtriplett got it you have mentioned changing the error message.