<rdar://problem/10605072>

Added the ability to override command line commands. In some cases GUI interfaces
might want to intercept commands like "quit" or "process launch" (which might cause
the process to re-run). They can now do so by overriding/intercepting commands
by using functions added to SBCommandInterpreter using a callback function. If the
callback function returns true, the command is assumed to be handled. If false
is returned the command should be evaluated normally.

Adopted this up in the Driver.cpp for intercepting the "quit" command.

llvm-svn: 151708