[dev.typeparams] cmd/compile/internal/types2: make Interface.obj a *TypeName

We know the exact type, so make it that. This saves some code
and a word of space with each Interface.

Follow-up on a comment in https://golang.org/cl/329309.

Change-Id: I827e39d17aae159a52ac563544c5e6d017bc05ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/332011
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/call.go

@@ -479,11 +479,10 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 			var why string
 			if tpar := asTypeParam(x.typ); tpar != nil {
 				// Type parameter bounds don't specify fields, so don't mention "field".
-				switch obj := tpar.Bound().obj.(type) {
-				case nil:
+				if tname := tpar.Bound().obj; tname != nil {
+					why = check.sprintf("interface %s has no method %s", tname.name, sel)
+				} else {
 					why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
-				case *TypeName:
-					why = check.sprintf("interface %s has no method %s", obj.name, sel)
 				}
 			} else {
 				why = check.sprintf("type %s has no field or method %s", x.typ, sel)

src/cmd/compile/internal/types2/sizeof_test.go

@@ -28,7 +28,7 @@ func TestSizeof(t *testing.T) {
 		{Tuple{}, 12, 24},
 		{Signature{}, 44, 88},
 		{Union{}, 24, 48},
-		{Interface{}, 44, 88},
+		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
 		{Named{}, 84, 160},

src/cmd/compile/internal/types2/type.go

@@ -264,7 +264,7 @@ func (s *Signature) Variadic() bool { return s.variadic }
 
 // An Interface represents an interface type.
 type Interface struct {
-	obj       Object        // type name object defining this interface; or nil (for better error messages)
+	obj       *TypeName     // corresponding declared object; or nil (for better error messages)
 	methods   []*Func       // ordered list of explicitly declared methods
 	embeddeds []Type        // ordered list of explicitly embedded elements
 	embedPos  *[]syntax.Pos // positions of embedded elements; or nil (for error messages) - use pointer to save space