Silence gcc's -Wcomment

Silence gcc's -Wcomment

gcc's (4.7, I think) -Wcomment warning is not "as smart" as clang's and
warns even if the line right after the backslash-newline sequence only has
a line comment that starts at the beginning of the line.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220360 91177308-0d34-0410-b5e6-96231b3b80d8