AK: Add InitializerList, an implementation of std::initializer_list

Use the AK version of std::initializer_list in AK::Vector, but only
when in serenity. When building AK for a non-serenity target, the header
<initializer_list> should be always available.

AK/InitializerList.h

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2020, Andrew Kaster <andrewdkaster@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+// std::initializer_list is a special type as far as the compiler is concerned.
+// As far as why it needs implemented in the library... blame the committee
+
+namespace std {
+
+template<class E>
+class initializer_list {
+public:
+    typedef E value_type;
+    typedef const E& reference;
+    typedef const E& const_reference;
+    typedef size_t size_type;
+    typedef const E* iterator;
+    typedef const E* const_iterator;
+
+    constexpr initializer_list() noexcept = default;
+    constexpr size_t size() const noexcept { return m_size; };
+    constexpr const E* begin() const noexcept { return m_begin; };
+    constexpr const E* end() const noexcept { return m_begin + m_size; }
+
+private:
+    // Note: begin has to be first or the compiler gets very upset
+    const E* m_begin = { nullptr };
+    size_t m_size = { 0 };
+
+    // The compiler is allowed to call this constructor
+    constexpr initializer_list(const E* e, size_t s) noexcept
+        : m_begin(e)
+        , m_size(s)
+    {
+    }
+};
+
+template<class E>
+constexpr const E* begin(initializer_list<E> il) noexcept
+{
+    return il.begin();
+}
+
+template<class E>
+constexpr const E* end(initializer_list<E> il) noexcept
+{
+    return il.end();
+}
+
+} // end namespace std

AK/Vector.h

@@ -33,15 +33,11 @@
 #include <AK/Traits.h>
 #include <AK/kmalloc.h>
 
-// NOTE: We can't include <initializer_list> during the toolchain bootstrap,
-//       since it's part of libstdc++, and libstdc++ depends on LibC.
-//       For this reason, we don't support Vector(initializer_list) in LibC.
-#ifndef SERENITY_LIBC_BUILD
-#    include <initializer_list>
-#endif
-
-#ifndef __serenity__
+#ifdef __serenity__
+#    include <AK/InitializerList.h>
+#else // !__serenity__
 #    include <new>
+#    include <initializer_list>
 #endif
 
 namespace AK {
@@ -144,14 +140,12 @@ public:
         clear();
     }
 
-#ifndef SERENITY_LIBC_BUILD
     Vector(std::initializer_list<T> list)
     {
         ensure_capacity(list.size());
         for (auto& item : list)
             unchecked_append(item);
     }
-#endif
 
     Vector(Vector&& other)
         : m_size(other.m_size)