[FIR] Properly collect overriddens for method enhancement

If some java class has multiple supertypes then we need to collect
  overriddens from all those types directly, even if superTypeScope
  (which is FirTypeIntersectionScope in this case) returns only
  one symbol from one of this types (not intersection one)

This is needed to proper enhancement in cases when some type occurs
  multiple times in supertypes graph with different nullability
  of arguments:

class ConcurrentHashMap<K, V> : AbstractMap<K!, V!>, MutableMap<K, V>

If we try to find method `get(key: K): V` supertype scope returns
  `AbstractMap.get(key: K!): V!` (because it actually overrides
  `MutableMap(key: K): V?`), but we need to get both symbols to
  properly enhance types for `ConcurrentHashMap.remove`

compiler/testData/diagnostics/testsWithStdLib/purelyImplementedCollection/mapsWithNullableKey.kt

@@ -17,6 +17,7 @@ fun hashMapTest() {
     x[bar()] = 1
     x[""] = <!TYPE_MISMATCH!>nullableInt<!>
     x[""] = 1
+    x[""] = <!NULL_FOR_NONNULL_TYPE!>null<!>
 
     val b1: MutableMap<String?, Int?> = <!TYPE_MISMATCH!>x<!>
     val b2: MutableMap<String?, Int> = x