Comment parsing: -fparse-all-comments: recognize empty line comments

In -fparse-all-comments mode empty '//' comments were recognized as
RCK_Invalid, and were not merged with next and previous lines.

Patch by Amin Shali.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180625 91177308-0d34-0410-b5e6-96231b3b80d8