<rdar://problem/12523238> Commit 1 of 3

This commit enables the new HasChildren() feature for synthetic children providers
Namely, it hooks up the required bits and pieces so that individual synthetic children providers can implement a new (optional) has_children call
Default implementations have been provided where necessary so that any existing providers continue to work and behave correctly

Next steps are:
2) writing smart implementations of has_children for our providers whenever possible
3) make a test case

llvm-svn: 166495