Mention 2015 edition in "note: `?` is not a macro repetition operator" errors

[SimonSapin]

Example error message:

error: expected `*` or `+`
   --> src/libcore/macros.rs:767:41
    |
767 |             impl $( <$( $lifetime ),+> )? Fn<(&'a str,)> for $Name {
    |                                         ^
    |
    = note: `?` is not a macro repetition operator

But it is! I couldn’t figure it out without asking on IRC what’s going on. It is a repetition operator… but not in the 2015 edition. Switching to 2018 would make this error go away.

The error message should mention this.

[estebank]

Mentoring notes:

There are three notes in https://github.com/rust-lang/rust/blob/master/src/libsyntax/ext/tt/quoted.rs that need to be updated to read

`?` is not a macro repetition operator in the 2015 edition, but accepted in the 2018 edition

After doing the code change you need to run ./x.py test src/test/ui --stage 1 --bless to update the stderr output files. The Forge has many extra resources to read, particularly useful is "How to build and run".