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.