ADT: Remove redundant `alignas` from IntervalMap, NFC

`AlignedArrayCharUnion` is now using `alignas`, which is properly
supported now by all the host toolchains we support. As a result, the
extra `alignas` on `IntervalMap` isn't needed anymore.

This is effectively a revert of 379daa29.

Differential Revision: https://reviews.llvm.org/D92509