Sanitize names for backing fields and private declarations in JS BE

See KT-1816

js/js.translator/testData/box/expression/identifierClash/privateDeclarations.kt

@@ -0,0 +1,17 @@
+// EXPECTED_REACHABLE_NODES: 1000
+open class A {
+    private val `.` = "A"
+    private val `;` = "B"
+
+    private fun `@`() = "C"
+    private fun `#`() = "D"
+
+    fun foo() = `.` + `;` + `@`() + `#`()
+}
+
+fun box(): String {
+    val x = A().foo()
+    if (x != "ABCD") return "fail: $x"
+
+    return "OK"
+}