cmd/gc: improve "type *X has no field or method M" message when X is an interface type

[Sajmani]

When using a new, unfamiliar package, programmers occasionally make the error of using a pointer to an interface type instead of the interface type directly:
http://play.golang.org/p/JulyJVaOrG
The compiler reports the error:
/tmp/sandbox956919287/main.go:8: x.Do undefined (type *ImportedFromSomewhere has no field or method Do)
While this is correct, it would be helpful to point out that *ImportedFromSomewhere is a pointer to an interface type, and better yet, that ImportedFromSomewhere has the desired method Do.

[gopherbot]

CL https://golang.org/cl/9731 mentions this issue.

[rsc]

Just for the record (because the CL will be implemented this way), I disagree with the 'better yet'. If you're using a pointer to interface, reporting that fact is enough. That's a day 1 Go mistake, better to focus on that issue than cloud things by saying "and you were this close!"