Fix a problem where the stop-hook command 'frame variable g_val' produces nothing

when newly created threads were subsequently stopped due to breakpoint hit.
The stop-hook mechanism delegates to CommandInterpreter::HandleCommands() to
execuet the commands.  Make sure the execution context is switched only once
at the beginning of HandleCommands() only and don't update the context while looping
on each individual command to be executed.

rdar://problem/10228156

llvm-svn: 141144