Clear global entity usage information when irg entity usage is cleared.

This fixes opt/fehler260.c.
irmemory: cleanup, remove unused alias result caching mechanism
irmemory cleanups/improvements
builtin_may_alias arguments do not count as address_taken
more irmemory improvements/cleanups
move classify API to firm private, remove unused language disambiguator option
irmemory.c: interpret reference modes as different from int modes
irmemory: rewrite classify logic

We can now differentiate between objects with compound type and
primitive type.