go/ssa: complete interface types for correct string form (fix build)

Fixes golang/go#22933.

Change-Id: I0cf0aab8b77114ee7eb1e181aee2c5b75087aa83
Reviewed-on: https://go-review.googlesource.com/80897
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/internal/gccgoimporter/parser_test.go

@@ -47,6 +47,11 @@ func TestTypeParser(t *testing.T) {
 			t.Errorf("expected full parse, stopped at %q", p.lit)
 		}
 
+		// interfaces must be explicitly completed
+		if ityp, _ := typ.(*types.Interface); ityp != nil {
+			ityp.Complete()
+		}
+
 		got := typ.String()
 		if got != test.want {
 			t.Errorf("got type %q, expected %q", got, test.want)

go/ssa/builder.go

@@ -58,7 +58,7 @@ var (
 	tString     = types.Typ[types.String]
 	tUntypedNil = types.Typ[types.UntypedNil]
 	tRangeIter  = &opaqueType{nil, "iter"} // the type of all "range" iterators
-	tEface      = new(types.Interface)
+	tEface      = types.NewInterface(nil, nil).Complete()
 
 	// SSA Value constants.
 	vZero = intConst(0)