ADT: Stop peeking inside AlignedCharArrayUnion, NFC
Update all the users of AlignedCharArrayUnion to stop peeking inside
(to look at buffer) so that a follow-up patch can replace it with an
alias to std::aligned_union_t.
This was reviewed as part of https://reviews.llvm.org/D92512, but I'm
splitting this bit out to commit first to reduce churn in case the
change to AlignedCharArrayUnion needs to be reverted for some