Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/testsWithStdLib/elvisOnJavaList.fir.kt

@@ -15,5 +15,5 @@ public class P {
 
 fun foo(c: P): MutableList<Int> {
     // Error should be here: see KT-8168 Typechecker fails for platform collection type
-    return c.getList() ?: listOf()
+    return <!RETURN_TYPE_MISMATCH!>c.getList() ?: listOf()<!>
 }

compiler/testData/diagnostics/testsWithStdLib/when/kt10807.fir.kt

@@ -5,9 +5,9 @@ import kotlin.comparisons.nullsLast
 class Foo(val a: String, val b: Int)
 
 fun getComp(): Comparator<Foo?> =
-        when {
+        <!RETURN_TYPE_MISMATCH!>when {
             else -> nullsLast(compareBy({ it.<!UNRESOLVED_REFERENCE!>a<!> }, { it.<!UNRESOLVED_REFERENCE!>b<!> }))
-        }
+        }<!>
 
 fun getCompInverted(): Comparator<Foo?> =
         nullsLast(