Add a (Python only) lldb.SBSyntheticValueProvider class to our API surface
On a suggestion from Jim Ingham, this class allows you to very easily define synthetic child providers that return a synthetic value (in the sense of r219330), but no children

Also, document this new feature in our www docs



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219337 91177308-0d34-0410-b5e6-96231b3b80d8