Pull up AssemblyWriter interface into header to allow subclassing
- made all functions virtual so that subclasses can specialize them
- add printInstructionLine so that subclasses can choose whether or not to
  print the newline character (without having to implement printBasicBlock()
- added a second constructor to AssemblyWriter that does not require a
  SlotTracker, as required in order to keep the SlotTracker helper class outside
  AsmWriter.h and buried in the implementation.