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

See KT-19772

compiler/testData/diagnostics/testsWithJsStdLib/wrongMultipleInheritance.kt

@@ -0,0 +1,25 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+open class A {
+    fun get(index: Int): Char = '*'
+}
+
+abstract class <!WRONG_MULTIPLE_INHERITANCE!>B<!> : A(), CharSequence
+
+interface I {
+    fun nextChar(): Char
+}
+
+abstract class <!WRONG_MULTIPLE_INHERITANCE!>C<!> : CharIterator(), I {
+    override fun nextChar(): Char = '*'
+}
+
+class <!WRONG_MULTIPLE_INHERITANCE!>CC(val s: CharSequence)<!> : CharSequence by s, MyCharSequence {}
+
+interface MyCharSequence {
+    val length: Int
+
+    operator fun get(index: Int): Char
+
+    fun subSequence(startIndex: Int, endIndex: Int): CharSequence
+}