<rdar://problem/13341472>

LLDB wasn't printing the names for negative enums. Fixed the signed extraction of enumerators and how they were registered with clang's type system.
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>