Auto merge of #6807 - anall:feature/use_new_diagnostics, r=Manishearth

migrate paths to newly-added diagnostic items

This gets rid of the following paths:
  * OS_STRING
  * TO_OWNED
  * TO_STRING

Removes some usages of:
 * PATH_BUF

Per #5393

also removes unneeded `is_ty_param_path` from `clippy_lints::types` and relocates `is_ty_param_lang_item` and `is_ty_param_diagnostic_item` to `clippy_utils`.

changelog: none