KT-25907: Use proper loop parameter type in for-in-CharSequence

compiler/testData/codegen/box/controlStructures/forNullableCharInString.kt

@@ -0,0 +1,12 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    val str = "abcd"
+    var r = ""
+    for (c: Char? in str) {
+        r = r + c ?: "?"
+    }
+    if (r != "abcd") throw AssertionError()
+
+    return "OK"
+}