[UB] Don't allocate space for contained types and then try to copy the

[UB] Don't allocate space for contained types and then try to copy the
contained types into the space when we have no contained types. This
fixes the UB stemming from a call to memcpy with a null pointer. This
also reduces the calls to allocate because this actually happens in
a notable client - Clang.

Found by UBSan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243944 91177308-0d34-0410-b5e6-96231b3b80d8