Preprocessor: don't keep comments under -traditional-cpp.

This patch is designed for minimal intrusion into normal preprocessing
and compilation; under -E -traditional-cpp, the lexer will still
generate tok::comment nodes since it is preserving all whitespace, but
the output printer will then throw it away.

<rdar://problem/13338680>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176534 91177308-0d34-0410-b5e6-96231b3b80d8