Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEach.fir.kt

@@ -13,5 +13,5 @@ fun list(start: SomeObject): SomeObject {
         e = e<!UNSAFE_CALL!>.<!>next()
     }
     // Smart cast is not possible here due to next()
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEachWithBreak.fir.kt

@@ -14,5 +14,5 @@ fun list(start: SomeObject): SomeObject {
         e.doSomething()
         e = e.next()
     }
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEachWithContinue.fir.kt

@@ -14,5 +14,5 @@ fun list(start: SomeObject): SomeObject {
         e.doSomething()
         e = e.next()
     }
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }