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 379daa2974.

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

llvm/include/llvm/ADT/IntervalMap.h

@@ -963,8 +963,7 @@ public:
 
 private:
   // The root data is either a RootLeaf or a RootBranchData instance.
-  alignas(RootLeaf) alignas(RootBranchData)
-      AlignedCharArrayUnion<RootLeaf, RootBranchData> data;
+  AlignedCharArrayUnion<RootLeaf, RootBranchData> data;
 
   // Tree height.
   // 0: Leaves in root.