[lldb] Fix NSDate test after Scalar change

The formatter was requesting an unsigned integer from the ValueObject,
but CFAbsoluteTime is a signed double, so in the NSDate test the formatter
actually just printed the 'error value' date which is the Cocoa epoch. This
started failing after the recent Scalar changes.

This patch just changes the logic to use a signed value which fits to the data
we try to read and avoids this issue.