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

See KT-19772

js/js.translator/testData/box/coercion/bridgeChar.kt

@@ -0,0 +1,41 @@
+// EXPECTED_REACHABLE_NODES: 1039
+open class A {
+    fun foo(): Char = 'X'
+}
+
+interface I {
+    fun foo(): Any
+}
+
+class B : A(), I
+
+fun typeOf(x: dynamic): String = js("typeof x")
+
+fun box(): String {
+    val b = B()
+    val i: I = B()
+    val a: A = B()
+
+    val r1 = typeOf(b.asDynamic().foo())
+    if (r1 != "object") return "fail1: $r1"
+
+    val r2 = typeOf(i.asDynamic().foo())
+    if (r2 != "object") return "fail2: $r2"
+
+    val r3 = typeOf(a.asDynamic().foo())
+    if (r3 != "object") return "fail3: $r3"
+
+    val x4 = b.foo()
+    val r4 = typeOf(x4)
+    if (r4 != "number") return "fail4: $r4"
+
+    val x5 = i.foo()
+    val r5 = typeOf(x5)
+    if (r5 != "object") return "fail5: $r5"
+
+    val x6 = a.foo()
+    val r6 = typeOf(x6)
+    if (r6 != "number") return "fail6: $r6"
+
+    return "OK"
+}