[FIR] Use the proper processAll* functions for CONFLICTING_OVERLOADS

The `*leaf*` functions were
introduced due to my false
assumption. The normal processing
functions only collect leafs
already. `*leaf*`s are bad, because
they don't cache anything.

The change in
AbstractFirOverrideScope reflects
KT-63290, and is needed to avoid
duplicate `CONFLICTING_OVERLOADS`
and `VIRTUAL_MEMBER_HIDDEN`.

compiler/testData/diagnostics/testsWithJvmBackend/duplicateJvmSignature/finalMembersFromBuiltIns/enumMembers.fir.kt

@@ -1,6 +0,0 @@
-enum class E {
-    ENTRY;
-
-    fun <!VIRTUAL_MEMBER_HIDDEN!>getDeclaringClass<!>() {}
-    <!ACCIDENTAL_OVERRIDE!>fun <!VIRTUAL_MEMBER_HIDDEN!>finalize<!>() {}<!>
-}

compiler/testData/diagnostics/testsWithJvmBackend/duplicateJvmSignature/finalMembersFromBuiltIns/enumMembers.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 enum class E {
     ENTRY;