cmd/compile: better diagnostics of wrong type in type switch

[dvyukov]

The program is:
http://play.golang.org/p/7lZfmkosub

When I assign X to I, compiler nicely says "X does not implement I (Foo method has pointer receiver)". However, in type switch it says just "i (type I) cannot have dynamic type X (missing Foo method)" for the same error. It would be nice to say "(Foo method has pointer receiver)" in type switch as well.

[gopherbot]

CL https://golang.org/cl/35235 mentions this issue.