Rollup merge of #131403 - practicalrs:fix_needless_lifetimes_p2, r=petrochenkov

Fix needless_lifetimes in rustc_serialize

Hi,

This PR fixes the following clipy warnings:

```
warning: the following explicit lifetimes could be elided: 'a
   --> compiler/rustc_serialize/src/serialize.rs:328:6
    |
328 | impl<'a, S: Encoder, T: Encodable<S>> Encodable<S> for Cow<'a, [T]>
    |      ^^                                                    ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
    |
328 - impl<'a, S: Encoder, T: Encodable<S>> Encodable<S> for Cow<'a, [T]>
328 + impl<S: Encoder, T: Encodable<S>> Encodable<S> for Cow<'_, [T]>
    |

warning: the following explicit lifetimes could be elided: 'a
   --> compiler/rustc_serialize/src/serialize.rs:348:6
    |
348 | impl<'a, S: Encoder> Encodable<S> for Cow<'a, str> {
    |      ^^                                   ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
help: elide the lifetimes
    |
348 - impl<'a, S: Encoder> Encodable<S> for Cow<'a, str> {
348 + impl<S: Encoder> Encodable<S> for Cow<'_, str> {
    |

warning: the following explicit lifetimes could be elided: 'a
   --> compiler/rustc_serialize/src/serialize.rs:355:6
    |
355 | impl<'a, D: Decoder> Decodable<D> for Cow<'a, str> {
    |      ^^                                   ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
help: elide the lifetimes
    |
355 - impl<'a, D: Decoder> Decodable<D> for Cow<'a, str> {
355 + impl<D: Decoder> Decodable<D> for Cow<'_, str> {
```

Best regards,
Michal

Author: matthiaskrgr