This patch does a few things:
- adds a new flag to mark ValueObjects as "synthetic children generated"
- vends new Create functions as part of the SyntheticChildrenFrontEnd that set the flag automatically
- moves synthetic child providers over to using these new functions

No visible feature change, but preparatory work for feature change



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223819 91177308-0d34-0410-b5e6-96231b3b80d8