report an error for unrecognized backend options
reset anonymous entity when parsing types in contexts not involving declarations

This should fix firm-testsuite/C/anon_entity.c
cleanup casts to bool
on assignments use bitfield base type as lhs type

Note that for the result of the whole expression or when bitfields are
used as values we still have to use int as type.

This simplifies the enum conversion warning and should fix
firm-testsuite/boolbitfield.c because of the implicit cast to bool.