Try to replace/simplify `apply_post_deduced_return_type` using `apply_deduced_return_type`

[JAChapmanII]

When the DECL_POST_FN of a deduced return type function is initially generated, there is an undeduced auto type variable passed as an argument to the post fn. Once it's fully deduced, we need to install the correct parameter type and rebuild the function type through apply_post_deduced_return_type. See if more of this can be shared with the similar logic apply_deduced_return_type used for the actual function.