[dev.typeparams] cmd/compile: handle meth expressions on typeparams

Rewrite a method expression such as 'T.String' (where T is type param
and String is part of its type bound Stringer) as:

func(rcvr T, other params...) {
    return Stringer(rcvr).String(other params...)
}

New function buildClosure2 to create the needed closure. The conversion
Stringer(rcvr) uses the dictionary in the outer function.

For a method expression like 'Test[T].finish' (where finish is a method
of Test[T]), we can already deal with this in buildClosure(). We just
need fix transformDot() to allow the method lookup to fail, since shapes
have no methods on them. That's fine, since for any instantiated
receiver type, we always use the methods on the generic base type.

Also removed the OMETHEXPR case in the main switch of node(), which
isn't needed any (and removes one more potential unshapify).

Also, fixed two small bugs with handling closures that have generic
params or generic captured variables. Need to set the instInfo for the
closure in the subst struct when descending into a closure during
genericSubst() and was missing initializing the startItabConv and gfInfo
fields in the closure info.

Change-Id: I6dadedd1378477936a27c9c544c014cd2083cfb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/338129


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>