gccrs: derive: Add proper derive(Clone) for unions

gccrs: derive: Add proper derive(Clone) for unions

gcc/rust/ChangeLog:

	* ast/rust-ast-builder.cc (AstBuilder::struct_expr_struct): New function.
	(AstBuilder::let): Likewise.
	(AstBuilder::struct_expr): Likewise.
	(AstBuilder::struct_expr_field): Likewise.
	(AstBuilder::field_access): Likewise.
	(AstBuilder::wildcard): Likewise.
	* ast/rust-ast-builder.h: Likewise.
	* expand/rust-derive-clone.cc (DeriveClone::visit_union): Implement
	properly.

gcc/testsuite/ChangeLog:

	* rust/compile/derive_macro4.rs: New test.
	* rust/compile/derive_macro6.rs: New test.