Do not point at whole file missing fn main

Only point at the end of the crate. We could try making it point at the
beginning of the crate, but that is confused with `DUMMY_SP`, causing
the output to be *worse*.

This change will make it so that VSCode will *not* underline the whole
file when `main` is missing, so other errors will be visible.

Author: estebank

tests/ui/crashes/ice-6250.stderr

@@ -1,14 +1,8 @@
 error[E0601]: `main` function not found in crate `ice_6250`
-  --> $DIR/ice-6250.rs:4:1
+  --> $DIR/ice-6250.rs:16:2
    |
-LL | / pub struct Cache {
-LL | |     data: Vec<i32>,
-LL | | }
-LL | |
-...  |
-LL | |     }
-LL | | }
-   | |_^ consider adding a `main` function to `$DIR/ice-6250.rs`
+LL | }
+   |  ^ consider adding a `main` function to `$DIR/ice-6250.rs`
 
 error[E0308]: mismatched types
   --> $DIR/ice-6250.rs:12:14

tests/ui/crashes/ice-6251.stderr

@@ -1,10 +1,8 @@
 error[E0601]: `main` function not found in crate `ice_6251`
-  --> $DIR/ice-6251.rs:4:1
+  --> $DIR/ice-6251.rs:6:2
    |
-LL | / fn bug<T>() -> impl Iterator<Item = [(); { |x: [u8]| x }]> {
-LL | |     std::iter::empty()
-LL | | }
-   | |_^ consider adding a `main` function to `$DIR/ice-6251.rs`
+LL | }
+   |  ^ consider adding a `main` function to `$DIR/ice-6251.rs`
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
   --> $DIR/ice-6251.rs:4:45