KT-2752: add diagnostic that checks whether Kotlin declarations produce conflicting names in generated JS

compiler/testData/diagnostics/testsWithJsStdLib/name/propertyAndMethodInSubclass.kt

@@ -0,0 +1,9 @@
+package foo
+
+open class Super {
+    <!JS_NAME_CLASH!>val foo = 23<!>
+}
+
+class Sub : Super() {
+    <!JS_NAME_CLASH!>fun foo() = 42<!>
+}