Debugger: check that label for marked object is a valid java identifier (KT-12651)

#KT-12651 Fixed

idea/testData/debugger/tinyApp/src/evaluate/singleBreakpoint/labels/lIdentifier.kt

@@ -0,0 +1,20 @@
+package lIdentifier
+
+fun main(args: Array<String>) {
+    val a = 1
+    val b = 1
+    val c = 1
+    val d = 1
+    val e = 1
+    //Breakpoint!
+    val f = 1
+}
+
+// DEBUG_LABEL: a = 0
+// DEBUG_LABEL: b = <no_name>
+// DEBUG_LABEL: c = for
+// DEBUG_LABEL: d = *
+// DEBUG_LABEL: e = e-e
+
+// EXPRESSION: a
+// RESULT: 1: I