[SmallVector] Add default initialization of SmallVectorStorage::InlineElts

[maksimsab]

Summary

This change mitigates a Coverity diagnostic about reading uninitialized memory (https://cwe.mitre.org/data/definitions/457.html).
The following snippet demonstrates the case.

SmallVector<int, 4> func(int a) {

if (a == 0) {
  return {};  // To get around RVO.
}

SmallVector<int, 4> ret;
return ret;

}

[nikic]

This memory should certainly not be initialized.

llvm/include/llvm/ADT/SmallVector.h

SmallVectorImpl<T> &SmallVectorImpl<T>::operator=(SmallVectorImpl<T> &&RHS) {
   RHS.clear();
   return *this;
 }
 
 /// Storage for the SmallVector elements.  This is specialized for the N=0 case
 /// to avoid allocating unnecessary storage.
 template <typename T, unsigned N>
 struct SmallVectorStorage {
-  alignas(T) char InlineElts[N * sizeof(T)];
+  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(T) SmallVectorStorage<T, 0> {};
 
 /// Forward declaration of SmallVector so that