Turn off 'quit' confirmation in lldb-mi

Summary:
# Turn off interpreter.prompt-on-quit on startup (MI)
# Add CommandInterpreter::SetPromptOnQuit
# Add SBCommandInterpreter::GetPromptOnQuit/SetPromptOnQuit

All tests pass on OS X.

Test Plan:
```
-file-exec-and-symbols ~/p/hello
-break-insert -f main
-exec-run
-interpreter-exec console quit
```

Reviewers: abidh, clayborg

Reviewed By: abidh, clayborg

Subscribers: lldb-commits, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D8444

llvm-svn: 233034