FIR checker: introduce DECLARATION_SIGNATURE_OR_DEFAULT positioning strategy

and fix CONFLICTING_OVERLOADS to use it

compiler/testData/diagnostics/tests/dataClasses/conflictingCopyOverloads.fir.kt

@@ -1,6 +1,6 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 data class A(val x: Int, val y: String) {
-    <!CONFLICTING_OVERLOADS!>fun copy(x: Int, y: String) = x<!>
-    <!CONFLICTING_OVERLOADS!>fun copy(x: Int, y: String) = A(x, y)<!>
+    <!CONFLICTING_OVERLOADS!>fun copy(x: Int, y: String)<!> = x
+    <!CONFLICTING_OVERLOADS!>fun copy(x: Int, y: String)<!> = A(x, y)
 }