[FIR] Only check conflicting callables that may clash with declared ones

This change improves performance, as
checking callables via scopes in
`4e587157` turns out to be quite
slow, and it also prevents some redundant
diagnostics.

compiler/testData/diagnostics/tests/delegation/clashes/finalMemberOverridden.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class Final {
     fun foo() {}
     val bar: Int = 0