[ADT] Force the alignment of the `data` field of `IntervalMap`

Summary:
This patch forces the alignment of the `data` field of `IntervalMap`.
It is because x86 MSVC doesn't apply automatically
(without `__declspec(align(...))`) alignments more than 4 bytes,
even if `alignof` has returned so. Consider the example:

https://godbolt.org/z/zIPa_G

Here `alignof` for both `S0` and `S1` returns `8`, but only `S1` is really
aligned on x86. The explanation of this behavior is here:

https://docs.microsoft.com/en-us/cpp/build/conflicts-with-the-x86-compiler

Reviewers: bkramer, stoklund, hans, rnk

Reviewed By: rnk

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D52613

llvm-svn: 344027