gccrs: Support outer attribute handling on trait items just like normal items

gccrs: Support outer attribute handling on trait items just like normal items

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-base.h (class ItemWrapper): New class.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::handle_outer_attributes): Use
	`ItemWrapper` class.
	(ASTLoweringBase::handle_doc_item_attribute): Likewise.
	(ASTLoweringBase::handle_lang_item_attribute): Likewise.
	* hir/rust-ast-lower-implitem.h: Check outer attributes on items.
	* hir/tree/rust-hir-item.h: Add `get_trait_locus` methods.
	* hir/tree/rust-hir.h: Likewise.
	* util/rust-hir-map.h: Add defId mappings and associated functions.
	* util/rust-hir-map.cc (Mappings::insert_defid_mapping): Implement insertion to said
	mappings.
	(Mappings::lookup_trait_item_defid): And looking up said mappings.