[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.kt

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