gccrs: expand: Move derive system to new one

gccrs: expand: Move derive system to new one

Builtin derive already had their own code scheme, incompatible with the
proc macro pattern. This commit unifies derive macros with it.

gcc/rust/ChangeLog:

	* ast/rust-ast-fragment.cc (Fragment::Fragment): Remove
	overwrite member in constructor.
	(Fragment::operator=): Removal of overwrite member in copy.
	(Fragment::should_overwrite): Remove overwrite getter.
	* ast/rust-ast-fragment.h: Remove overwrite boolean member.
	* expand/rust-expand-visitor.cc (derive_item): Add a function to
	derive items using the expander for now.
	(builtin_derive_item): Rename from derive_item to
	builtin_derive_item.
	(ExpandVisitor::visit): Remove visit to derive attributes.
	(ExpandVisitor::expand_derive): Remove derive attribute visitor.
	(ExpandVisitor::visit_attrs_with_derive): Likewise.
	* expand/rust-expand-visitor.h: Update insertion of other kind
	of proc macros. We no longer have an overwrite attribute in the
	fragment.
	* expand/rust-macro-expand.cc (MacroExpander::parse_procmacro_output):
	Return the fragment instead of inserting it.
	* expand/rust-macro-expand.h (struct MacroExpander): Return
	fragment.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>