[ADT] Use alignas + sizeof for inline storage, NFC

AlignedCharArrayUnion is really only needed to handle the "union" case
when we need memory of suitable size and alignment for multiple types.
SmallVector only needs storage for one type, so use that directly.

include/llvm/ADT/SmallVector.h

@@ -14,7 +14,6 @@
 #define LLVM_ADT_SMALLVECTOR_H
 
 #include "llvm/ADT/iterator_range.h"
-#include "llvm/Support/AlignOf.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
@@ -97,8 +96,9 @@ using SmallVectorSizeType =
 
 /// Figure out the offset of the first element.
 template <class T, typename = void> struct SmallVectorAlignmentAndSize {
-  AlignedCharArrayUnion<SmallVectorBase<SmallVectorSizeType<T>>> Base;
-  AlignedCharArrayUnion<T> FirstEl;
+  alignas(SmallVectorBase<SmallVectorSizeType<T>>) char Base[sizeof(
+      SmallVectorBase<SmallVectorSizeType<T>>)];
+  alignas(T) char FirstEl[sizeof(T)];
 };
 
 /// This is the part of SmallVectorTemplateBase which does not depend on whether
@@ -870,13 +870,13 @@ SmallVectorImpl<T> &SmallVectorImpl<T>::operator=(SmallVectorImpl<T> &&RHS) {
 /// to avoid allocating unnecessary storage.
 template <typename T, unsigned N>
 struct SmallVectorStorage {
-  AlignedCharArrayUnion<T> InlineElts[N];
+  alignas(T) char InlineElts[N * sizeof(T)];
 };
 
 /// We need the storage to be properly aligned even for small-size of 0 so that
 /// the pointer math in \a SmallVectorTemplateCommon::getFirstEl() is
 /// well-defined.
-template <typename T> struct alignas(alignof(T)) SmallVectorStorage<T, 0> {};
+template <typename T> struct alignas(T) SmallVectorStorage<T, 0> {};
 
 /// This is a 'vector' (really, a variable-sized array), optimized
 /// for the case when the array is small.  It contains some number of elements

include/llvm/ADT/iterator_range.h

@@ -18,7 +18,6 @@
 #ifndef LLVM_ADT_ITERATOR_RANGE_H
 #define LLVM_ADT_ITERATOR_RANGE_H
 
-#include <iterator>
 #include <utility>
 
 namespace llvm {