[BOLT] Fix debug info update for zero-length ranges.

Summary:
Due to a clowntown on my part we were generating wrong ranges
when an empty range was seen on input. We were basically expanding
the range to include all basic blocks following such range and setting
wrong sizes at the same time.

Add "-dump-cu" option to llvm-dwarfdump that allows to look at debug
info of a single compile unit only. Saves time if we are only interested
in a subset of information.

(cherry picked from FBD4430989)