Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/borrow/struct_variant.stderr

@@ -1,8 +1,9 @@
 error: #[serde(borrow)] may only be used on newtype variants
- --> $DIR/struct_variant.rs:7:10
-  |
-7 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+  --> $DIR/struct_variant.rs:9:5
+   |
+9  | /     #[serde(borrow)]
+10 | |     S { s: Str<'a> },
+   | |____________________^
 
 error: aborting due to previous error