go/types: add test verifying corner-case behavior of iota

For #22341.

Change-Id: I7e586c58075e4c835ef97701db2f38bdf7558521
Reviewed-on: https://go-review.googlesource.com/71974
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/types/api_test.go

@@ -118,6 +118,8 @@ func TestValuesInfo(t *testing.T) {
 		{`package f7a; var _ complex128 = -1e-2000i`, `-1e-2000i`, `complex128`, `(0 + 0i)`},
 		{`package f6b; var _            =  1e-2000i`, `1e-2000i`, `complex128`, `(0 + 0i)`},
 		{`package f7b; var _            = -1e-2000i`, `-1e-2000i`, `complex128`, `(0 + 0i)`},
+
+		{`package g0; const (a = len([iota]int{}); b; c); const _ = c`, `c`, `int`, `2`}, // issue #22341
 	}
 
 	for _, test := range tests {