Split .debug_info section to compilation units.

Split .debug_info section to compilation units.

This resolves old TODO in the code.  It is also very important
for performance and memory usage of any tools which might be
consuming the debug data.  Without this split, the tools
would be forced to decode (and probably store in memory) all of
debug data even for a simple query.  With this change it should
be easy to find only the relevant compilation unit.

Change-Id: I9f285eaa01632b6adea39cae866df6f76e9d0062