attribs: Use attribute_value_equal in decl_attributes

attribs: Use attribute_value_equal in decl_attributes

This is another thing discussed in the 1/9 Reflection thread,
also not dependent on reflection.

decl_attributes calls simple_cst_equal on TREE_VALUEs of the
current and preexisting attributes, but that is just a small
part of how attribute values should be compared.

The following patch fixes that.

2025-12-06  Jakub Jelinek  <jakub@redhat.com>

	* attribs.cc (decl_attributes): Use attribute_value_equal to
	compare attribute values instead of simple_cst_equal.