Mention editions when emitting 2015 exclusive errors

[estebank]

The difference between edition defaults for new projects between cargo and rustc can lead to confusion, particularly for newcomers. All errors that would only happen on one edition should mention the discrepancy. We made an effort to have lints for 2015 to 2018 migration, but we were more lax on clarifying diagnostics when following 2018 guides when rustc is ran under 2015. Things like nll discrepancies (seen in #61855, moot point once we enable nll in 2015) come to mind, while things like let async = 6; (pass in 2015, "error: expected pattern, found reserved keyword async" without any mention of editions in 2018) which do not mention editions at all (it should recommend using r#raw_idents always, explain it was accepted in 2015 edition but it's being run in 2018).

[jebrosen]

As another example, forgetting or not knowing to move a project to the 2018 edition before adopting async/await currently causes error messages such as these:

error: expected identifier, found `3`
 --> src/main.rs:2:21
  |
2 |     let x = async { 3 };
  |             -----   ^ expected identifier
  |             |
  |             while parsing this struct

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, or an operator, found `move`
 --> src/main.rs:3:19
  |
3 |     let y = async move { 3 };
  |                   ^^^^ expected one of 7 possible tokens here

error[E0422]: cannot find struct, variant or union type `async` in this scope
 --> src/main.rs:2:13
  |
2 |     let x = async { 3 };
  |             ^^^^^ not found in this scope

error: aborting due to 3 previous errors