E0512 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0512.rs

E0512 needs a span_label, updating it from:

error[E0512]: transmute called with differently sized types: u16 (16 bits) to u8 (8 bits)
  --> src/test/compile-fail/E0512.rs:14:23
   |
14 |     unsafe { takes_u8(::std::mem::transmute(0u16)); } //~ ERROR E0512
   |                       ^^^^^^^^^^^^^^^^^^^^^

to:

error[E0512]: transmute called with differently sized types: u16 (16 bits) to u8 (8 bits)
  --> src/test/compile-fail/E0512.rs:14:23
   |
14 |     unsafe { takes_u8(::std::mem::transmute(0u16)); } //~ ERROR E0512
   |                       ^^^^^^^^^^^^^^^^^^^^^ transmuting between 16 bits and 8 bits

[bstrie]

Freeing this one up as part of the final push for #35233.

[gavinb]

I wrote a patch for this but then found that it had already been done in 157208b. So this issue can be closed.