<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 Depending on what happened stack frames could get out of date and a stale shared pointer (one that is no longer a current frame in a thread) could end up being used.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.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@177208 91177308-0d34-0410-b5e6-96231b3b80d8