ADT: Migrate users of AlignedCharArrayUnion to std::aligned_union_t, NFC

Prepare to delete `AlignedCharArrayUnion` by migrating its users over to
`std::aligned_union_t`.

I will delete `AlignedCharArrayUnion` and its tests in a follow-up
commit so that it's easier to revert in isolation in case some
downstream wants to keep using it.

Differential Revision: https://reviews.llvm.org/D92516