E0493 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0493.rs

E0493 needs a span_label, updating it from:

error[E0493]: constants are not allowed to have destructors
  --> src/test/compile-fail/E0493.rs:19:17
   |
19 | const F : Foo = Foo { a : 0 }; //~ ERROR E0493
   |                 ^^^^^^^^^^^^^

To:

error[E0493]: constants are not allowed to have destructors
  --> src/test/compile-fail/E0493.rs:19:17
   |
19 | const F : Foo = Foo { a : 0 }; //~ ERROR E0493
   |                 ^^^^^^^^^^^^^ constants cannot have destructors

Bonus: show a label where the destructor is defined:

error[E0493]: constants are not allowed to have destructors
  --> src/test/compile-fail/E0493.rs:19:17
   |
16 |     fn drop(&mut self) {}
   |     --------------------- destructor defined here
...
19 | const F : Foo = Foo { a : 0 }; //~ ERROR E0493
   |                 ^^^^^^^^^^^^^ constants cannot have destructors