Introduce FirFunctionReturnTypeMismatchChecker

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/45.fir.kt

@@ -17,5 +17,5 @@ fun case_1(x: Number?): Long? {
  * ISSUES: KT-22997
  */
 fun case_2(x: Number?): Long? {
-    if (x == null || x is Long) return x else return 0L
+    if (x == null || x is Long) return <!RETURN_TYPE_MISMATCH!>x<!> else return 0L
 }

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/64.fir.kt

@@ -17,7 +17,7 @@ class Case1<T> {
             x = getT()
         }
         <!DEBUG_INFO_EXPRESSION_TYPE("T?")!>x<!>
-        return x
+        return <!RETURN_TYPE_MISMATCH!>x<!>
     }
 }