When finding an invalid outer doc comment, suggest turning it into a comment

[estebank]

error: expected one of `.`, `;`, `?`, `}`, or an operator, found doc comment `//!self.allow_ty_infer()`
    --> src/librustc_typeck/astconv.rs:2461:21
     |
2460 |                     false
     |                          - expected one of `.`, `;`, `?`, `}`, or an operator
2461 |                     //!self.allow_ty_infer()
     |                     ^^^^^^^^^^^^^^^^^^^^^^^^ unexpected token

should be

error: expected one of `.`, `;`, `?`, `}`, or an operator, found doc comment `//!self.allow_ty_infer()`
    --> src/librustc_typeck/astconv.rs:2461:21
     |
2460 |                     false
     |                          - expected one of `.`, `;`, `?`, `}`, or an operator
2461 |                     //!self.allow_ty_infer()
     |                     ---^^^^^^^^^^^^^^^^^^^^^ unexpected token
     |                     |
     |                     help: if you wanted a regular comment, add a space before the exclamation mark: `// !`