[libc++][Modules] Add missing __fwd includes

A few __fwd includes are missing from public modules that will become noticeable when the private submodules are split into their own top level modules (D144322). Add the missing includes.

Reviewed By: ldionne, philnik, #libc

Differential Revision: https://reviews.llvm.org/D153216

libcxx/include/__memory_resource/memory_resource.h

@@ -11,6 +11,7 @@
 
 #include <__availability>
 #include <__config>
+#include <__fwd/memory_resource.h>
 #include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

libcxx/include/array

@@ -118,6 +118,7 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 #include <__algorithm/swap_ranges.h>
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/array.h>
 #include <__iterator/reverse_iterator.h>
 #include <__tuple/sfinae_helpers.h>
 #include <__type_traits/conditional.h>

libcxx/include/tuple

@@ -207,6 +207,7 @@ template <class... Types>
 #include <__config>
 #include <__functional/invoke.h>
 #include <__fwd/array.h>
+#include <__fwd/get.h>
 #include <__fwd/tuple.h>
 #include <__memory/allocator_arg_t.h>
 #include <__memory/uses_allocator.h>