Revert "Revert "Add support for fetching signed values from tagged pointers.""

This reverts commit 602ab188.

The patch replicated an lldbassert for a certain type of NSNumber for tagged
pointers.  This really shouldn't be an assert since we don't do anything wrong
with these numbers, we just don't print a summary.  So this patch changed the
lldbassert to a log message in reverting the revert.