Fixed an lldb infrastructure bug, where the debugger should reaaly update its

execution context only when the process is still alive.  When running the test
suite, the debugger is launching and killing processes constantly.

This might be the cause of the test hang as reported in rdar://problem/8377854,
where the debugger was looping infinitely trying to update a supposedly stale
thread list.

llvm-svn: 113022