[FIR] Don't remove subsumed members from intersection overrides's overriddens

This fixes a bunch of missing overridden symbols in IR.
This is also required for fixing KT-59921 in the following commit
where we need to keep all overridden symbols of intersection overrides
so that we can enhance them properly.

#KT-57300 Fixed
#KT-57299 Fixed
#KT-59921
#KT-57300
#KT-62788
#KT-64271
#KT-64382

compiler/testData/diagnostics/testsWithStdLib/purelyImplementedCollection/customClassMutableList.fir.kt

@@ -27,8 +27,8 @@ fun foo() {
     x.add(bar())
     x.add("")
 
-    x[0] = null
-    x[0] = bar()
+    x[0] = <!NULL_FOR_NONNULL_TYPE!>null<!>
+    x[0] = <!ARGUMENT_TYPE_MISMATCH!>bar()<!>
     x[0] = ""
 
     val b1: MutableList<String?> = <!INITIALIZER_TYPE_MISMATCH!>x<!>