Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/smartCasts/safecalls/falseSecondArgument.fir.kt

@@ -3,5 +3,5 @@
 fun calc(x: List<String>?, y: Int?): Int {
     x?.subList(y!! - 1, y)
     // y!! above should not provide smart cast here
-    return y
+    return <!RETURN_TYPE_MISMATCH!>y<!>
 }

compiler/testData/diagnostics/tests/smartCasts/safecalls/receiverAndChainFalse.fir.kt

@@ -6,5 +6,5 @@ fun calc(x: String?, y: Int?): Int {
     // Smart cast because of x!! in receiver
     foo(x!!)?.subSequence(y!!, x.length)?.length
     // No smart cast possible
-    return y
+    return <!RETURN_TYPE_MISMATCH!>y<!>
 }