<rdar://problem/13194155>

Variables view out of sync with lldb in Xcode is now fixed. Depending on what happened stack frames could get out of date and a stale shared pointer (one that is no longer a current frame in a thread) could end up being used. 

Now we don't store a weak_ptr to a frame in the ExecutionContextRef class, we just store its stack ID and we always regrab the frame from the thread by stack ID.

llvm-svn: 177208