Redundant E0106 missing lifetime error and rust_2018_idioms missing lifetime lint

[estebank]

While hacking in the compiler I got the following error:

error: hidden lifetime parameters in types are deprecated
   --> src/libsyntax_ext/format.rs:113:43
    |
113 |     arg_with_formatting_spans: Vec<(Span, parse::FormatSpec)>,
    |                                           ^^^^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
    |
note: lint level defined here
   --> src/libsyntax_ext/lib.rs:5:9
    |
5   | #![deny(rust_2018_idioms)]
    |         ^^^^^^^^^^^^^^^^
    = note: #[deny(elided_lifetimes_in_paths)] implied by #[deny(rust_2018_idioms)]

error[E0106]: missing lifetime specifier
   --> src/libsyntax_ext/format.rs:113:43
    |
113 |     arg_with_formatting_spans: Vec<(Span, parse::FormatSpec)>,
    |                                           ^^^^^^^^^^^^^^^^^ expected lifetime parameter

The errors are correct (and the lint is even helpful!) but they are 1) redundant and 2) E0106 is not as helpful as the rust_2018_idioms lint. We should get them both to parity and hide one if the other has triggered.