The SBValue impl class's GetSP can now fetch the dynamic type or the synthetic
children - which it may have to compute.  Thus it needs to take the API lock.

<rdar://problem/13560869>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178734 91177308-0d34-0410-b5e6-96231b3b80d8