Basic support for reading synthetic children by index:

 if your datatype provides synthetic children, "frame variable object[index]" should now do the right thing
 in cases where the above syntax would have been rejected before, i.e.
  object is not a pointer nor an array (frame variable ignores potential overload of [])
  object is a pointer to an Objective-C class (which cannot be dereferenced)
 expression will still run operator[] if available and complain if it cannot do so
 synthetic children by name do not work yet

llvm-svn: 137097