Rollup merge of #54458 - scottmcm:bug-54456, r=nikomatsakis

Allow both explicit and elided lifetimes in the same impl header

While still prohibiting explicit and in-band in the same header.

Fixes #54456

As usual, I don't know the broader context of the code I'm changing, so please let me know whatever I can do better.

Pre-existing test that mixing explicit and in-band remains an error: https://github.com/rust-lang/rust/blob/master/src/test/ui/in-band-lifetimes/E0688.rs

src/test/ui/impl-header-lifetime-elision/explicit-and-elided-same-header.rs

@@ -0,0 +1,25 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+
+#![allow(warnings)]
+
+#![feature(impl_header_lifetime_elision)]
+
+// This works for functions...
+fn foo<'a>(x: &str, y: &'a str) {}
+
+// ...so this should work for impls
+impl<'a> Foo<&str> for &'a str {}
+trait Foo<T> {}
+
+fn main() {
+}