Fix "//" comments with -traditional-cpp in C++.

Apparently, gcc's -traditional-cpp behaves slightly differently in C++ mode;
specifically, it discards "//" comments.  Match gcc's behavior.

<rdar://problem/14808126>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189515 91177308-0d34-0410-b5e6-96231b3b80d8