<rdar://problem/12523238> Commit 2 of 3

Adding the new has_children (or MightHaveChildren() in C++) for the existing synthetic children providers
In a few cases, the new call is going to be much more efficient than the previous num_children > 0 check
When the optimization was marginal (e.g. std::vector<>), the choice was to use num_children in order to keep
implementation details in one function instead of duplicating code

Next step is to provide test cases

llvm-svn: 166506