auto merge of #4911 : lifthrasiir/rust/comment-exemptions, r=catamorphism

This is a natural extension of #4887, and handles the following three cases:

~~~~
a line with only /s
////////////////////////////////////////////

a line with only /s followed by whitespace
////////////////////////////////////////////

a block comment with only *s between two /s
/********************************/
~~~~

diff --cc src/libsyntax/parse/comments.rs
Simple merge

diff --cc src/libsyntax/parse/lexer.rs
Simple merge