Make Redex build on clang without requiring C++17 support
Make Redex build on clang without requiring C++17 support

Summary:
1. Clang only added support for `aligned_alloc()` with C++17 and only on MacOS 10.15. This diff replaces the usage of `aligned_alloc()` with a special class that provides the same functionality.
2. Added `override` annotations where needed.

Reviewed By: int3

Differential Revision: D18452309

fbshipit-source-id: 39747f9194735ae75541a7192a5acb2e187e0c64

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