[lldb/DataFormatters] Fix the `$$deference$$` synthetic child

Summary:
The ValueObject code checks for a special `$$dereference$$` synthetic
child to allow formatter providers to implement a natural
dereferencing behavior in `frame variable` for objects like smart
pointers.

This support was broken when used directly throught the Python API and
not trhough `frame variable`. The reason is that
SBFrame.FindVariable() will return by default the synthetic variable
if it exists, while `frame variable` will not do this eagerly. The
code in `ValueObject::Dereference()` accounted for the latter but not
for the former. The fix is trivial. The test change includes
additional covergage for the already-working bahevior as it wasn't
covered by the testsuite before.

This commit also adds a short piece of documentatione explaining that
it is possible (even advisable) to provide this synthetic child
outstide of the range of the normal children.

Reviewers: jingham

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D73053