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

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 4d8bf870a82765eb0d4fe53c82f796b957c05954.

This reverts commit d10f9863a5ac1cb681af07719650c44b48f289ce.

This reverts commit 4b5dc150b9862271720b3d56a3e723a55dd81838.