gccrs: derive(PartialEq): Implement proper discriminant comparison

gccrs: derive(PartialEq): Implement proper discriminant comparison

And use the new EnumMatchBuilder class to do so.

gcc/rust/ChangeLog:

	* expand/rust-derive-partial-eq.cc (DerivePartialEq::eq_fn): Change signature.
	(DerivePartialEq::visit_tuple): Use new eq_fn API.
	(DerivePartialEq::visit_struct): Likewise.
	(DerivePartialEq::visit_enum): Implement proper discriminant comparison.
	* expand/rust-derive-partial-eq.h: Change eq_fn signature.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/derive-partialeq2.rs: Add declaration for
	discriminant_value.