gccrs: derive(Clone): Implement derive clone for enum struct variants

gccrs: derive(Clone): Implement derive clone for enum struct variants

gcc/rust/ChangeLog:

	* expand/rust-derive-clone.cc (DeriveClone::clone_enum_struct): New function for deriving
	enum struct variants.
	(DeriveClone::visit_enum): Call into the new function.

gcc/testsuite/ChangeLog:

	* rust/compile/nr2/exclude:
	* rust/compile/derive_clone_enum1.rs: New test.
	* rust/compile/derive_clone_enum2.rs: New test.
	* rust/compile/derive_clone_enum3.rs: New test.
	* rust/execute/torture/derive_clone_enum1.rs: New test.