gccrs: ast: Refactor TraitItem to keep Location info

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Keep location in TraitItem base class
	* ast/rust-item.h (class TraitItemFunc): Use base class location instead.
	(class TraitItemMethod): Likewise.
	(class TraitItemConst): Likewise.
	(class TraitItemType): Likewise.
	* ast/rust-macro.h: Likewise.