FIR checker: fix positions of type mismatch on overrides

To handle implicit return types, those should be reported on
declarations with return type positioning strategy, instead of
return type itself.

compiler/testData/diagnostics/tests/delegatedProperty/delegatedPropertyOverridedInTraitTypeMismatch.fir.kt

@@ -1,21 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER
-
-import kotlin.reflect.KProperty
-
-interface A {
-    val prop: Int
-}
-
-class AImpl: A  {
-    override val prop by Delegate()
-}
-
-fun foo() {
-    AImpl().prop
-}
-
-class Delegate {
-    operator fun getValue(t: Any?, p: KProperty<*>): <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>String<!> {
-        return ""
-    }
-}

compiler/testData/diagnostics/tests/delegatedProperty/delegatedPropertyOverridedInTraitTypeMismatch.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 import kotlin.reflect.KProperty