[FIR] UNREACHABLE_CODE diagnostic (wip)

Implementation for PSI only

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCallInReceiver.fir.kt

@@ -1,11 +1,11 @@
 fun test11() {
     fun Any.bar(i: Int) {}
-    todo().bar(1)
+    todo()<!UNREACHABLE_CODE!>.bar(1)<!>
 }
 
 fun test12() {
     fun Any.bar(i: Int) {}
-    todo()<!UNNECESSARY_SAFE_CALL!>?.<!>bar(1)
+    todo()<!UNREACHABLE_CODE!><!UNNECESSARY_SAFE_CALL!>?.<!>bar(1)<!>
 }
 
 fun todo(): Nothing = throw Exception()