[FIR] Unwrap substitution overrides for multiple default values

Otherwise, duplicate diagnostics may be reported on
subtypes of the type that actually declares the
problematic override.

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndSubstitutionOverrides.fir.kt

@@ -1,15 +0,0 @@
-interface A<T> {
-    fun foo(x: Int = 42): Int
-}
-
-open class B<K> {
-    fun foo(x: Int = 239) = x
-}
-
-interface C<E> {
-    fun foo(x: Int): Int
-}
-
-open <!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE!>class Z<!><R> : A<R>, B<R>(), C<R>
-
-<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION_WARNING!>class N<!><W> : Z<W>()

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndSubstitutionOverrides.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A<T> {
     fun foo(x: Int = 42): Int
 }