Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/callableReference/generic/expectedFunctionType.fir.kt

@@ -19,6 +19,6 @@ class A3<T> {
     fun test2(): (T) -> Unit = A3<T>()::a3
     fun test3(): (Int) -> String = A3<Int>()::a3
 
-    fun <R> test4(): (R) -> Unit = this::a3
+    fun <R> test4(): (R) -> Unit = <!RETURN_TYPE_MISMATCH!>this::a3<!>
     fun <R> test5(): (T) -> R = this::a3
 }

compiler/testData/diagnostics/tests/callableReference/unitAdaptationForReferenceCompatibility.fir.kt

@@ -20,6 +20,6 @@ public interface Executor {
 fun f(): String = "test"
 
 class A {
-    fun schedule1(e: Executor): Future<String> = e.submit(::f)
-    fun schedule2(e: Executor): Future<String> = e.submit { f() }
+    fun schedule1(e: Executor): Future<String> = <!RETURN_TYPE_MISMATCH!>e.submit(::f)<!>
+    fun schedule2(e: Executor): Future<String> = <!RETURN_TYPE_MISMATCH!>e.submit { f() }<!>
 }