FIR: use only most specific members when building intersection

In this commit we drop less concrete members in intersection.
See for example change in test data: before the commit, we intersected
all three A.foo, B.foo and C.foo in diamond hierarchy. After the commit
we drop A.foo and intersect B.foo and C.foo only.

compiler/testData/diagnostics/tests/override/FakeOverrideModality3.fir.kt

@@ -1,16 +0,0 @@
-interface A {
-    fun foo() {}
-}
-
-interface B : A {
-    abstract override fun foo()
-}
-
-interface C : A {
-    abstract override fun foo()
-}
-
-interface D : A
-
-// Fake override Z#foo should be abstract
-<!MANY_INTERFACES_MEMBER_NOT_IMPLEMENTED!>class Z<!> : B, C, D

compiler/testData/diagnostics/tests/override/FakeOverrideModality3.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A {
     fun foo() {}
 }