Improve support of simple bit-fields in -fdump-ada-spec

Improve support of simple bit-fields in -fdump-ada-spec

This fixes an issue with nested structures and adds an Alignment clause
to counter the effect of the Pack aspect.

gcc/c-family/ChangeLog:
	* c-ada-spec.c (dump_nested_type) <RECORD_TYPE>: Remove obsolete code.
	(dump_ada_structure): Also deal with convention, unchecked union and
	bit-field for nested types.  In the latter case, print an Alignment
	aspect along with the Pack aspect.