stdlib: type-erased collections: add forwarding for '_customContainsEquatableElement()'

Author: gribozavr

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -667,6 +667,12 @@ extension Any${Kind} {
       whereSeparator: isSeparator)
   }
 
+  public func _customContainsEquatableElement(
+    _ element: Element
+  ) -> Bool? {
+    return _box.__customContainsEquatableElement(element)
+  }
+
   public func _preprocessingPass<R>(
     _ preprocess: () throws -> R
   ) rethrows -> R? {

validation-test/stdlib/ExistentialCollection.swift.gyb

@@ -183,6 +183,10 @@ tests.test("${TestedType}: dispatch to wrapped") {
     _ = s.split { (_) in true }
   }
 
+  Log._customContainsEquatableElement.expectIncrement(Base.self) {
+    _ = s._customContainsEquatableElement(OpaqueValue(42))
+  }
+
   Log._preprocessingPass.expectIncrement(Base.self) {
     _ = s._preprocessingPass {}
   }