This function is used to refine the location of warning 21 "this statement never returns (or has an unsound type.", e.g. to avoid pointing to an expression of the form let ... = .... in e (but point to e instead). In this PR:

• The traversal is done in the resulting Typedtree, not the original Parsetree. This removes one ad hoc traversal of the Parsetree (see New implementation of Env.make_copy_of_types #8774 for some contexts of why this could be useful).
• The traversal is done only when the warning needs to be raised. This removes a quadratic behavior (ok, a cheap one) on long sequences.
• Local opens/modules bindings/exception definitions are traversed as well.