Add an API on ValueObject to generate a 'synthetic child' of base class type. Note that in this commit, the term synthetic child is not meant to refer to data formatters, but to the programmatically-generated children stored inside a ValueObject itself

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216483 91177308-0d34-0410-b5e6-96231b3b80d8