improve case with both anonymous lifetime parameters in functions and trait objects.

[gaurikholkar-zz]

An example of the above iss

fn foo(x:  fn(&u8, &u8), mut y: Vec<&u8>, z: &u8) {
  y.push(z); 
}

cc @nikomatsakis

[nikomatsakis]

So, this particular example is a special-case of #43269 -- but I think that there is an analogous version where the user already has given a name:

fn foo<'a>(x:  fn(&u8, &u8), mut y: Vec<&u8>, z: &'a u8) {
  y.push(z); 
}

fn main() { }

This seem to give the correct error message, though, so I guess we just need a test:

error[E0621]: explicit lifetime required in the type of `y`
 --> src/main.rs:2:10
  |
1 | fn foo<'a>(x:  fn(&u8, &u8), y: Vec<&u8>, z: &'a u8) {
  |                              - consider changing the type of `y` to `std::vec::Vec<&'a u8>`
2 |   y.push(z); 
  |          ^ lifetime `'a` required

[gaurikholkar-zz]

Done in #44516. Closing the issue.
cc @nikomatsakis