Implement interactive command interruption

The core of this change is the new CommandInterpreter::m_command_state,
which models the state transitions for interactive commands, including
an "interrupted" state transition.

In general, command interruption requires cooperation from the code
executing the command, which needs to poll for interruption requests
through CommandInterpreter::WasInterrupted().

CommandInterpreter::PrintCommandOutput() implements an optionally
interruptible printing of the command output, which for large outputs 
was likely the longest blocking part.
(ex. target modules dump symtab on a complex binary could take 10+ minutes)

Differential Revision: https://reviews.llvm.org/D37923

llvm-svn: 315037