FIR u/s scope: choose most specific intersection member for override check

Before this commit we took just first intersection member for this check.
However it's quite bad, because we were dependent on supertype order.
Choosing the most specific member looks more consistent here.

#KT-50969 Fixed

compiler/fir/providers/src/org/jetbrains/kotlin/fir/scopes/impl/AbstractFirUseSiteMemberScope.kt

@@ -130,7 +130,7 @@ abstract class AbstractFirUseSiteMemberScope(
              *
              * TODO: is it enough to check only one function?
              */
-            firstMember
+            mostSpecific
         } else {
             chosenSymbol
         }

compiler/fir/providers/src/org/jetbrains/kotlin/fir/scopes/impl/FirTypeIntersectionScopeContext.kt

@@ -73,8 +73,8 @@ class FirTypeIntersectionScopeContext(
                 ).member as D
             }
 
-            val firstMember: D
-                get() = context.extractedOverrides.first().member
+            val mostSpecific: D
+                get() = context.mostSpecific
         }
     }