ADT: Remove redundant `alignas` from IntervalMap, NFC

Authored by dexonsmith on Dec 2 2020, 1:48 PM.


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 379daa29744cd96b0a87ed0d4a010fa4bc47ce73.

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