[FIR] Implement checker for exhaustive when's in expression position

compiler/testData/diagnostics/tests/controlStructures/ifWhenToAnyComplexExpressions.fir.kt

@@ -11,7 +11,7 @@ fun testMixedIfAndWhen() =
                 else 1
                     true -> if (true) 42
                 else println()
-                else -> if (true) println()
+                else -> <!INVALID_IF_AS_EXPRESSION!>if<!> (true) println()
             }
         else println()
 
@@ -28,4 +28,4 @@ fun testWrappedExpressions() =
         }
         else {
             (((((42)) + 1)))
-        }
+        }