Rework how the API mutex is acquired when filling out an ExecutionContext from...

Rework how the API mutex is acquired when filling out an ExecutionContext from an ExecutionContextRef,
particularly in the SBThread & SBFrame interfaces.  Instead of filling the whole context & then getting
the API mutex, we now get only the target, acquire the API mutex from it, then fill out the rest of the
context.  This removes a race condition where you get a ThreadSP, then wait on the API mutex while another
command Destroy's the Thread you've just gotten.
Also fixed the ExecutionContextRef::Get*SP calls so they don't return invalid objects.
Also fixed the ExecutionContext::Has*Scope calls so they don't claim to have a scope if the object representing
that scope has been destroyed.
Also fixed a think-o in Thread::IsValid which was causing it to return the opposite of the desired value.

<rdar://problem/11995490>

llvm-svn: 162401