Improve error on temp value to point to the correct statement

[dtolnay]

The error used to point to a statement "inside" the macro invocation, rather than a real statement that makes sense from the user's point of view.

Before:

error[E0716]: temporary value dropped while borrowed
 --> $DIR/temporary-value.rs:4:22
  |
4 |     let _ = anyhow!(&String::new());
  |             ---------^^^^^^^^^^^^^-
  |             |        |
  |             |        creates a temporary which is freed while still in use
  |             temporary value is freed at the end of this statement
  |             argument requires that borrow lasts for `'static`

After:

error[E0716]: temporary value dropped while borrowed
 --> $DIR/temporary-value.rs:4:22
  |
4 |     let _ = anyhow!(&String::new());
  |             ---------^^^^^^^^^^^^^-- temporary value is freed at the end of this statement
  |             |        |
  |             |        creates a temporary which is freed while still in use
  |             argument requires that borrow lasts for `'static`