Change the mechanism around SBValue::GetSP() so that it always requires the target API lock AND the

process StopLocker (if there is a process) before it will hand out SBValues.  We were doing this in 
an ad hoc fashion previously, and then playing whack-a-mole whenever we found a place where we should
have been doing this but weren't.  Really, it doesn't make sense to be poking at SBValues when the target
is running, the dynamic and synthetic values can't really be computed, and the underlying memory may be
incoherent.

<rdar://problem/13819378> Sometimes when stepping fast, my inferior is killed by debugserver

llvm-svn: 181863