[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344027 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/IntervalMap.h

@@ -964,6 +964,7 @@ class IntervalMap {
 
 private:
   // The root data is either a RootLeaf or a RootBranchData instance.
+  LLVM_ALIGNAS(RootLeaf) LLVM_ALIGNAS(RootBranchData)
   AlignedCharArrayUnion<RootLeaf, RootBranchData> data;
 
   // Tree height.