[lldb-mi] Fix evaluation for children of created variable object.

Move code in CMICmdCmdVarListChildren::Execute() up so that the child
object will always be added when the MI command -var-list-children is
entered (instead of only when the print-value was all or simple).  This
patch fixes evaluation of expressions like varobj.member for a created
varobj with children.

Reviewed by: abidh
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D11659

llvm-svn: 243782