[DbgHistoryCalculator] Improve debug messages

I've found that a frequent source of debug information loss in optimized
code is due to DEBUG_VALUE intrinsics in a position of the instruction
stream that is outside the scope of the variable it describes.

Tracking these is pretty difficult with the existing debug messages of
the history calculator; this patch addresses the issue by making it
obvious when this event happens.

Differential Revision: https://reviews.llvm.org/D147718