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/testsWithStdLib/regression/ea66827_dataClassWrongToString.fir.kt

@@ -1,7 +0,0 @@
-data class A(val x: Int) {
-    fun toArray(): IntArray =
-            intArrayOf(x)
-
-    override fun toString() =
-            toArray().takeWhile { it != -1 } // .joinToString()
-}

compiler/testData/diagnostics/testsWithStdLib/regression/ea66827_dataClassWrongToString.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 data class A(val x: Int) {
     fun toArray(): IntArray =
             intArrayOf(x)