raw_ostream: << operator for callables with raw_stream argument

This allows easier construction of print helpers. Example:

Printable PrintLaneMask(unsigned LaneMask) {
  return Printable([LaneMask](raw_ostream &OS) {
    OS << format("%08X", LaneMask);
  });
}

// Usage:
OS << PrintLaneMask(Mask);

Differential Revision: http://reviews.llvm.org/D14348

llvm-svn: 254655