[lldb][DataFormatter] Add dereference support to libstdcpp std::shared_ptr formatter

This mimicks the implementation of the libstdcpp std::unique_ptr
formatter.

This has been attempted several years ago in
`0789722d` but was reverted in
`e7dd3972`.

The difference to the original patch is that we now maintain
a `$$dereference$$` member and we only store weak pointers
to the other children inside the synthetic frontend. This is
what the libc++ formatters do to prevent the recursion mentioned
in the revert commit.