Merge -fstandalone-debug from Clang r198655:

  Implement a new -fstandalone-debug option. rdar://problem/15685848
  It controls everything that -flimit-debug-info used to, plus the
  vtable type optimization. The old -fno-limit-debug-info option is now an
  alias to -fstandalone-debug and vice versa.

  Standalone is the default on Darwin until dtrace is updated to work with
  non-standalone debug info (rdar://problem/15758808).

  Note: I kept the LimitedDebugInfo name in CodeGenOptions::DebugInfoKind
  because NoStandaloneDebugInfo sounded even more confusing.