Provide an easy way for synthetic child provider front ends to declare themselves “invalid”

This is not being used yet, and in practice, more refactoring would be required to make this fully practical
In practice, the way this should work is that CalculateNumChildren(), GetChildAtIndex(), GetIndexOfChildWithName() and MightHaveChildren() should all default to failure values when m_valid == false. Update() should be the only function actually setting/clearing the flag upon inspecting the backend ValueObject, if it determines it to be in an incongruent state
Given refactoring of the FrontEnd APIs, this work could be automatically performed without the individual providers having to replicate this logic
The way this works now is that each front end picks one or more “key ivars” and keys off those to detect invalidity
This is a baby step 0 to a better world

llvm-svn: 196452

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