ADT: Rely on std::aligned_union_t for math in AlignedCharArrayUnion, NFC

Instead of computing the alignment and size of the `char` buffer in
`AlignedCharArrayUnion`, rely on the math in `std::aligned_union_t`.
Because some users of this rely on the `buffer` field existing with a
type convertible to `char *`, we can't change the field type, but we can
still avoid duplicating the logic.

A potential follow up would be to delete `AlignedCharArrayUnion` after
updating its users to use `std::aligned_union_t` directly; or if we like
our template parameters better, could update users to stop peeking
inside and then replace the definition with:
```
template <class T, class... Ts>
using AlignedCharArrayUnion = std::aligned_union_t<1, T, Ts...>;
```

Differential Revision: https://reviews.llvm.org/D92500