Originally the requirement was f(x) where f's argument is
exactly x's type.

CL 11858043 relaxed the requirement in a non-standard
way: f's argument must be exactly x's type or interface{}.

If we're going to relax the requirement, it should be done
in a way consistent with the rest of Go. This CL allows f's
argument to have any type for which x is assignable;
that's the same requirement the compiler would impose
if compiling f(x) directly.

Fixes #5368.

R=dvyukov, bradfitz, pieter
CC=golang-dev
https://golang.org/cl/12895043