Tolerate DOS-style EOL in IDL line comments

If an IDL file uses DOS-style EOLs (CR+LF), line comments need to
ignore the second linebreak character, otherwise, as is currently
the case, the parsed documentation comment includes a trailing `\r`
character, which is then output verbatim into the output source
code by flatc.

Change-Id: I39591631995a980622d20a4a32315178b33f18f6