E0121 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0121.rs

Error E0121 needs a span_label, updating it from:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> src/test/compile-fail/E0121.rs:11:13
   |
11 | fn foo() -> _ { 5 } //~ ERROR E0121
   |             ^

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> src/test/compile-fail/E0121.rs:13:13
   |
13 | static BAR: _ = "test"; //~ ERROR E0121
   |             ^

To:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> src/test/compile-fail/E0121.rs:11:13
   |
11 | fn foo() -> _ { 5 } //~ ERROR E0121
   |             ^ not allowed in type signatures

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> src/test/compile-fail/E0121.rs:13:13
   |
13 | static BAR: _ = "test"; //~ ERROR E0121
   |             ^ not allowed in type signatures

[intrepion]

I'd like to take this