impl Trait creates conflicting duplicates of higher-ranked lifetimes in fn pointer types.

[eddyb]

Minimal testcase:

fn foo() -> impl Into<for<'a> fn(&'a ())> {
    |_| {} as for<'a> fn(&'a ())
}

error[E0261]: use of undeclared lifetime name `'a`
 --> src/main.rs:1:35
  |
1 | fn foo() -> impl Into<for<'a> fn(&'a ())> {
  |                                   ^^ undeclared lifetime

error[E0496]: lifetime name `'a` shadows a lifetime name that is already in scope
 --> src/main.rs:1:27
  |
1 | fn foo() -> impl Into<for<'a> fn(&'a ())> {
  |                           ^^      -- first declared here
  |                           |
  |                           lifetime 'a already in scope

Cause is known (mishandling in hir::lowering), using this issue to track the pending fix.