Fix ICE for mismatched args on target without span

[etaoins]

Commit 7ed00ca improved our error reporting by including the target function in our error messages when there is an argument count mismatch. A simple example from the UI tests is:

error[E0593]: function is expected to take a single 2-tuple as argument, but it takes 0 arguments
  --> $DIR/closure-arg-count.rs:32:53
   |
32 |     let _it = vec![1, 2, 3].into_iter().enumerate().map(foo);
   |                                                     ^^^ expected function that takes a single 2-tuple as argument
...
44 | fn foo() {}
   | -------- takes 0 arguments

However, this assumed the target span was always available. This does not hold true if the target function is in std or another crate. A simple example from #48046 is assigning str::split to a function type with a different number of arguments.

Fix by omitting all of the labels and suggestions related to the target span when it's not found.

Fixes #48046

r? @estebank

[estebank]

@bors r+ rollup

It'd be nice if the error pointed at the argument instead:

error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
  --> $DIR/closure-arg-count.rs:40:41
   |
40 |     let _it = vec![1, 2, 3].into_iter().map(usize::checked_add);
   |                                         ^^^ ------------------ takes 2 arguments
   |                                         |
   |                                         expected function that takes 1 argument

Filed #48099 for the above comment.

[bors]

📌 Commit daaa9a4 has been approved by estebank