typeck/pat.rs: extract `calc_default_binding_mode`.
typeck/pat.rs: extract `calc_default_binding_mode`.

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