[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/tests/j+k/purelyImplementedSupertype.fir.kt

@@ -0,0 +1,29 @@
+// FULL_JDK
+
+// FILE: Util.java
+public class Util {
+    public static String getString() { return null; }
+}
+
+// FILE: main.kt
+import java.util.concurrent.ConcurrentHashMap
+
+fun testWithMap(map: ConcurrentHashMap<Int, String>): Int {
+    var string = map[1]
+    if (string == null) {
+        string = map.computeIfAbsent(1) { "hello" }
+    }
+    return string.length
+}
+
+fun testWithUtil(map: ConcurrentHashMap<Int, String>): Int {
+    var string = map[1]
+    if (string == null) {
+        string = Util.getString()
+    }
+    return string<!UNSAFE_CALL!>.<!>length
+}
+
+fun test(list: java.util.ArrayList<String?>) {
+    val x = list.get(0)<!UNSAFE_CALL!>.<!>length
+}

compiler/testData/diagnostics/tests/j+k/purelyImplementedSupertype.kt

@@ -0,0 +1,29 @@
+// FULL_JDK
+
+// FILE: Util.java
+public class Util {
+    public static String getString() { return null; }
+}
+
+// FILE: main.kt
+import java.util.concurrent.ConcurrentHashMap
+
+fun testWithMap(map: ConcurrentHashMap<Int, String>): Int {
+    var string = map[1]
+    if (string == null) {
+        string = map.computeIfAbsent(1) { "hello" }
+    }
+    return <!DEBUG_INFO_SMARTCAST!>string<!>.length
+}
+
+fun testWithUtil(map: ConcurrentHashMap<Int, String>): Int {
+    var string = map[1]
+    if (string == null) {
+        string = Util.getString()
+    }
+    return string<!UNSAFE_CALL!>.<!>length
+}
+
+fun test(list: java.util.ArrayList<String?>) {
+    val x = list.get(0)<!UNSAFE_CALL!>.<!>length
+}

compiler/testData/diagnostics/tests/j+k/purelyImplementedSupertype.txt

@@ -0,0 +1,15 @@
+package
+
+public fun test(/*0*/ list: java.util.ArrayList<kotlin.String?>): kotlin.Unit
+public fun testWithMap(/*0*/ map: java.util.concurrent.ConcurrentHashMap<kotlin.Int, kotlin.String>): kotlin.Int
+public fun testWithUtil(/*0*/ map: java.util.concurrent.ConcurrentHashMap<kotlin.Int, kotlin.String>): kotlin.Int
+
+public open class Util {
+    public constructor Util()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun getString(): kotlin.String!
+}