Fix determinism bug when mapping type names (#8956)

When we rewrite module types, we must associate the old type names and
indices with the new types they are being mapped to. The code for this
was previously sensitive to the iteration order over the old-to-new type
map because it modified the module's type names as it traversed them and
because visitation order determined which name was kept in cases where
multiple old types map to the same new type. Fix both problems by
keeping the lesser names and indices in the case of merges and by not
updating the module type names until after the new type names have been
fully populated.