Runtime: correct invalid C++ workaround

The placement new operator is only available when `<new>` has been
included.  Add the missing include to the header to allow us to get the
definition of the placement new allocator.  This allows us to remove the
workaround that was there for Windows, and should hopefully repair the
Android build as well.

Thanks to @grynspan for helping identify the underlying issue!

Author: compnerd

include/swift/Runtime/Atomic.h

@@ -95,14 +95,6 @@ struct aligned_alloc<Alignment_, true> {
     free(ptr);
 #endif
   }
-
-#if defined(_WIN32)
-  // FIXME: why is this even needed?  This is not permitted as per the C++
-  // standrd new.delete.placement (§17.6.3.4).
-  [[nodiscard]] void *operator new(std::size_t size, void *where) noexcept {
-    return ::operator new(size, where);
-  }
-#endif
 };
 
 /// The default implementation for swift::atomic<T>, which just wraps

stdlib/public/runtime/MetadataImpl.h

@@ -51,6 +51,7 @@
 #include "EnumImpl.h"
 
 #include <cstring>
+#include <new>
 #include <type_traits>
 
 namespace swift {