Account for type param in '_ constraint for returned trait suggestion

[estebank]

Given

rust/src/test/ui/issues/issue-16922.rs

Lines 1 to 6 in f7a1f97

	use std::any::Any;
	fn foo<T: Any>(value: &T) -> Box<dyn Any> {
	Box::new(value) as Box<dyn Any>
	//~^ ERROR cannot infer an appropriate lifetime
	}

we currently output

rust/src/test/ui/issues/issue-16922.stderr

Lines 1 to 12 in f7a1f97

	error[E0759]: cannot infer an appropriate lifetime
	--> $DIR/issue-16922.rs:4:14
	|
	LL | fn foo<T: Any>(value: &T) -> Box<dyn Any> {
	| -- this data with an anonymous lifetime `'_`...
	LL | Box::new(value) as Box<dyn Any>
	| ^^^^^ ...is captured here, requiring it to live as long as `'static`
	|
	help: to declare that the trait object captures data from argument `value`, you can add an explicit `'_` lifetime bound
	|
	LL | fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> {
	| ^^^^

The suggestion is incomplete and will not result in compilable code. Because the lifetime is coming from an argument of type T, which is an unconstrained type parameter, T itself will not have the needed lifetime and causes the 'static obligation that we're trying to reverse by suggesting '_. The appropriate changes to the code would be either fn foo<'a, T: Any + 'a>(value: &'a T) -> Box<dyn Any + 'a> or fn foo(value: &dyn Any) -> Box<dyn Any + '_>, depending on whether T appears in any other argument or not.

[estebank]

The suggested code works since 1.64.