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

We determined that the MSVC implementation of std::aligned* isn't suited
to our needs. It doesn't support 16 byte alignment or higher, and it
doesn't really guarantee 8 byte alignment. See
https://github.com/microsoft/STL/issues/1533

Also reverts "ADT: Change AlignedCharArrayUnion to an alias of std::aligned_union_t, NFC"

Also reverts "ADT: Remove AlignedCharArrayUnion, NFC" to bring back
AlignedCharArrayUnion.

This reverts commit 4d8bf870.

This reverts commit d10f9863.

This reverts commit 4b5dc150.