[FIR] UNREACHABLE_CODE diagnostic (wip)

Implementation for PSI only

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

@@ -1,15 +1,15 @@
 fun testPrefix() {
     operator fun Any.not() {}
-    !todo()
+    <!UNREACHABLE_CODE!>!<!>todo()
 }
 
 fun testPostfixWithCall(n: Nothing) {
     operator fun Nothing.inc(): Nothing = this
-    <!VAL_REASSIGNMENT!>n<!>++
+    <!VAL_REASSIGNMENT!>n<!><!UNREACHABLE_CODE!>++<!>
 }
 
 fun testPostfixSpecial() {
-    todo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    todo()<!UNNECESSARY_NOT_NULL_ASSERTION, UNREACHABLE_CODE!>!!<!>
 }
 
 fun todo(): Nothing = throw Exception()