go/types, types2: report error for invalid string(1 << s)

For #45114.
Fixes #45117.

Change-Id: I71d6650ae2c4c06952fce19959120f15f13c08a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/379256
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

test/fixedbugs/bug193.go

@@ -11,8 +11,6 @@ func main() {
 	ss := 1 << s
 	y1 := float64(ss)
 	y2 := float64(1 << s) // ERROR "shift"
-	// see issues #45114, #45117
-	// y3 := string(1 << s)  // DISABLED "shift"
-	y3 := 0
+	y3 := string(1 << s)  // ERROR "shift"
 	_, _, _, _, _ = s, ss, y1, y2, y3
 }