A method expression is of the form T.m where T is a type and m
is a method of that type. The spec restricted T essentially to
a type name. Both cmd/compile and go/types accepted any type
syntactically, and a method expression was really just a form
of a selector expression x.f where x denotes a type.

This CL removes the spec syntax restriction from MethodExpr
to match the actual implementation. It also moves MethodExpr
from Operand to PrimaryExpr, because that's what it is.

It still keeps the separate notion of MethodExpr even though
it looks just like a selector expresion, since a MethodExpr
must start with a type rather than a value, and the spec's
syntax expresses this bit of semantics via distinct productions
(e.g., conversions look like calls but also must start with
a type).

Fixes #9060.

Change-Id: Idd84655b5b4f85d7ee53ebf749f73f0414a05f4a
Reviewed-on: https://go-review.googlesource.com/73233Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>