cmd/compile, go/types: erroneously accepts method expressions with anonymous receiver type

[mdempsky]

The Go spec's grammar restricts method expressions to using either a named type or pointer to a named type:

MethodExpr    = ReceiverType "." MethodName .
ReceiverType  = TypeName | "(" "*" TypeName ")" | "(" ReceiverType ")" .

cmd/compile erroneously accepts this Go source:

package p
var _ = struct{ error }.Error

go/types erroneously accepts it too.

gccgo correctly rejects with the error message "method expression requires named type or pointer to named type"

/cc @griesemer @ianlancetaylor

[griesemer]

This is so closely related to #9060 that I'm going to close it as duplicate. I believe we should just accept this code and adjust the spec.