Inconsistent treatment of carriage return in // and //// comments

[qnighy]

Although // and //// are both regarded as non-doc-comments, there is a difference in treatment of carriage return.

That is,

// Non-doc-comment with bare CR: '^M'
//// Non-doc-comment with bare CR: '^M'
fn main () {}

where '^M' is a carriage return, fails at line 2.

At least rustc diagnoses a wrong message: "bare CR not allowed in doc-comment".

Tested on Rust 1.16.0.