JS: fix behaviour of char-returning functions with multiple inheritance

See KT-19772

js/js.translator/testData/box/char/topLevelCallables.kt

@@ -0,0 +1,32 @@
+// SKIP_MINIFICATION
+fun foo(): Char = '1'
+
+val p1: Char = '2'
+
+var p2: Char = '3'
+
+var p3: Char = '4'
+    get() = field + 1
+    set(value) {
+        field = value + 1
+    }
+
+fun box(): String {
+    var root = eval("_")
+
+    var r = typeOf(root.foo())
+    if (r !== "number") return "fail1: $r"
+
+    r = typeOf(root.p1)
+    if (r !== "number") return "fail2: $r"
+
+    r = typeOf(root.p2)
+    if (r !== "number") return "fail3: $r"
+
+    r = typeOf(root.p3)
+    if (r !== "number") return "fail4: $r"
+
+    return "OK"
+}
+
+fun typeOf(x: dynamic): String = js("typeof x")