Extend synthetic children to produce synthetic values (as in, those that GetValueAsUnsigned(), GetValueAsCString() would return)

The way to do this is to write a synthetic child provider for your type, and have it vend the (optional) get_value function.
If get_value is defined, and it returns a valid SBValue, that SBValue's value (as in lldb_private::Value) will be used as the synthetic ValueObject's Value

The rationale for doing things this way is twofold:

- there are many possible ways to define a "value" (SBData, a Python number, ...) but SBValue seems general enough as a thing that stores a "value", so we just trade values that way and that keeps our currency trivial
- we could introduce a new level of layering (ValueObjectSyntheticValue), a new kind of formatter (synthetic value producer), but that would complicate the model (can I have a dynamic with no synthetic children but synthetic value? synthetic value with synthetic children but no dynamic?), and I really couldn't see much benefit to be reaped from this added complexity in the matrix
On the other hand, just defining a synthetic child provider with a get_value but returning no actual children is easy enough that it's not a significant road-block to adoption of this feature

Comes with a test case

llvm-svn: 219330
diff --git a/lldb/source/Core/ValueObjectCast.cpp b/lldb/source/Core/ValueObjectCast.cpp
index 4f4f8cc..b20371b 100644
--- a/lldb/source/Core/ValueObjectCast.cpp
+++ b/lldb/source/Core/ValueObjectCast.cpp
@@ -102,7 +102,7 @@
         //m_value.SetContext (Value::eContextTypeClangType, clang_type);
         m_value.SetClangType (clang_type);
         SetAddressTypeOfChildren(m_parent->GetAddressTypeOfChildren());
-        if (clang_type.IsAggregateType ())
+        if (!CanProvideValue())
         {
             // this value object represents an aggregate type whose
             // children have values, but this object does not. So we