re PR c/4784 (Anonymous structs issues)

re PR c/4784 (Anonymous structs issues)

	PR c/4784
	* c-decl.c (detect_field_duplicates_hash): New.  Handle anonymous
	structures and unions recursively.
	(detect_field_duplicates): Move duplicate detection with a hash to
	detect_field_duplicates_hash.  Always use a hash if anonymous
	structures or unions are present.
	* doc/extend.texi (Unnamed Fields): Document that duplicate fields
	give errors.

testsuite:
	* gcc.dg/anon-struct-9.c: New test.

From-SVN: r159204