go.tools/go/types: clean up internal iota in all cases

R=dsymonds
CC=golang-dev
https://golang.org/cl/33570043

go/types/resolver.go

@@ -661,6 +661,7 @@ func (check *checker) constDecl(obj *Const, typ, init ast.Expr) {
 		if !isConstType(t) {
 			check.errorf(typ.Pos(), "invalid constant type %s", t)
 			obj.typ = Typ[Invalid]
+			check.iota = nil
 			return
 		}
 		obj.typ = t

go/types/testdata/const0.src

@@ -13,6 +13,11 @@ const c0 = x /* ERROR "not constant" */
 // typed constants must have constant types
 const _ interface /* ERROR invalid constant type */ {} = 0
 
+func _ () {
+	const _ interface /* ERROR invalid constant type */ {} = 0
+	for i := 0; i < 10; i++ {} // don't crash with non-nil iota here
+}
+
 // untyped constants
 const (
 	// boolean values