Fix an issue where LLDB would run out of stack space trying to decide if a...

Fix an issue where LLDB would run out of stack space trying to decide if a deeply nested child can be updated in the face of an invalid execution context

The issue is that a child can't really ask the root object, since this decision could actually hinge on whether a dynamic and/or synthetic value is present
To do this, make values vote lazily for whether they are willing to allow this, so that we can navigate up the chain without recursively invoking ourselves

Tentative fix for rdar://21949558

llvm-svn: 243077