hir: change HirIdValidator.hir_ids_seen to a set
hir: change HirIdValidator.hir_ids_seen to a set

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust