[dev.typeparams] cmd/compile: handle calling a method on a type param in stenciling

- Have to delay the extra transformation on methods invoked on a type
   param, since the actual transformation (including path through
   embedded fields) will depend on the instantiated type. I am currently
   doing the transformation during the stencil substitution phase. We
   probably should have a separate pass after noder2 and stenciling,
   which drives the extra transformations that were in the old
   typechecker.

 - We handle method values (that are not called) and method calls. We
   don't currently handle method expressions.

 - Handle type substitution in function types, which is needed for
   function args in generic functions.

 - Added stringer.go and map.go tests, testing the above changes
   (including constraints with embedded interfaces).

Change-Id: I3831a937d2b8814150f75bebf9f23ab10b93fa00
Reviewed-on: https://go-review.googlesource.com/c/go/+/290550
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/helpers.go

@@ -119,6 +119,16 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 	n := ir.NewCallExpr(pos, ir.OCALL, fun, args)
 	n.IsDDD = dots
 
+	if fun.Op() == ir.OXDOT {
+		if fun.(*ir.SelectorExpr).X.Type().Kind() != types.TTYPEPARAM {
+			base.FatalfAt(pos, "Expecting type param receiver in %v", fun)
+		}
+		// For methods called in a generic function, don't do any extra
+		// transformations. We will do those later when we create the
+		// instantiated function and have the correct receiver type.
+		typed(typ, n)
+		return n
+	}
 	if fun.Op() != ir.OFUNCINST {
 		// If no type params, still do normal typechecking, since we're
 		// still missing some things done by tcCall below (mainly