Introduce FirFunctionReturnTypeMismatchChecker

compiler/fir/analysis-tests/testData/resolve/covariantArrayAsReceiver.kt

@@ -11,7 +11,7 @@ interface KtLightMethod : PsiElement
 // With covariant array here we do not visit lambda (it.usage is KtLightMethod) below
 // Problem disappears if 'out' is removed
 fun <T> Array<out T>.filterNot(f: (T) -> Boolean): List<T> {
-    return this
+    return <!RETURN_TYPE_MISMATCH!>this<!>
 }
 
 fun <T> Array<T>.toList(): List<T>? = null

compiler/fir/analysis-tests/testData/resolve/diagnostics/returnTypeMismatchOnOverride.kt

@@ -57,7 +57,7 @@ open class J() : S() {
 }
 
 open class Base<T : X, Z : T> {
-    open fun kek(): Z = Z()
+    open fun kek(): Z = <!RETURN_TYPE_MISMATCH!>Z()<!>
 }
 
 open class GoodDerrived : Base<Y, W>() {

compiler/fir/analysis-tests/testData/resolve/expresssions/invoke/explicitReceiver2.kt

@@ -1,6 +1,6 @@
 
 class Bar {
-    operator fun invoke(): Foo { return this } // (1)
+    operator fun invoke(): Foo { return <!RETURN_TYPE_MISMATCH!>this<!> } // (1)
 
 }
 
@@ -11,7 +11,7 @@ fun x() {
 class Foo {
 
 
-    operator fun Bar.invoke(): Foo { return this } // (2)
+    operator fun Bar.invoke(): Foo { return <!RETURN_TYPE_MISMATCH!>this<!> } // (2)
 
     val x: Bar = Bar()
 

compiler/fir/analysis-tests/testData/resolve/expresssions/invoke/threeReceivers.kt

@@ -1,7 +1,7 @@
 
 
 class Bar {
-    fun FooBar.invoke(): Bar = this
+    fun FooBar.invoke(): Bar = <!RETURN_TYPE_MISMATCH!>this<!>
 }
 
 class Buz