cpp: Ensure that CR/LF conversion is done in #"".
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2077
Reported by @grubba
There's a risk that CR/LF pairs are kept verbatim by #"", and not converted to a plain LF. This is not good if code has passed through LF => CR/LF conversion.