Add "indexedVariables" to variables with lots of children.

Prior to this fix if we have a really large array or collection class, we would end up always creating all of the child variables for an array or collection class. If the number of children was very high this can cause delays when expanding variables. By adding the "indexedVariables" to variables with lots of children, we can keep good performance in the variables view at all times. This patch will add the "indexedVariables" key/value pair to any "Variable" JSON dictionairies when we have an array of synthetic child provider that will create more than 100 children.

We have to be careful to not call "uint32_t SBValue::GetNumChildren()" on any lldb::SBValue that we use because it can cause a class, struct or union to complete the type in order to be able to properly tell us how many children it has and this can be expensive if you have a lot of variables. By default LLDB won't need to complete a type if we have variables that are classes, structs o...