UBSAN "downcast of address" runtime error on `createSentinal` for `ilist`s

ubsan-compiled dxc will fail with errors like:

/home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/SparseBitVector.h:256:44: runtime error: downcast of address 0x7ffc07f29348 with insufficient space for an object of type 'llvm::ilist_traits<llvm::SparseBitVectorElement<128>>::Element' (aka 'SparseBitVectorElement<128U>')
0x7ffc07f29348: note: pointer points here
 00 00 00 00  00 00 00 00 00 00 00 00  18 ab 02 00 10 61 00 00  38 ab 02 00 10 61 00 00  68 ab 02 00
              ^ 
    #0 0x5572402d0e1d in createSentinel /home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/SparseBitVector.h:256:44
    #1 0x5572402d0e1d in provideInitialHead /home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/SparseBitVector.h:260:48
    #2 0x5572402d0e1d in iplist /home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/ilist.h:350:25
    #3 0x5572402d0e1d in ilist /home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/ilist.h:603:3
    #4 0x5572402d0e1d in SparseBitVector /home/amaiorano/src/external/DirectXShaderCompiler/include/llvm/ADT/SparseBitVector.h:437:3
    #5 0x5572402d0e1d in (anonymous namespace)::SparseBitVectorTest_TrivialOperation_Test::TestBody() /home/amaiorano/src/external/DirectXShaderCompiler/unittests/ADT/SparseBitVectorTest.cpp:18:21
<snip>

The reason this fails is that most implementations of ilist_traits<T>::createSentinal return a down-casted instance of a ilist_half_node<T> or ilist_node<T>, which although can be cast-down to T (since T derives from ilist_node), means we're returning a pointer to a sliced object of type T. And although we never dereference the pointer, this is still UB, which is why UBSAN flags it.

This issue was already flagged and addressed in LLVM by @dexonsmith, as can be seen by most of his changes here, where Duncan ended up rewriting the ilist system to be clearer, potentially more efficient, and non-UB. See Duncan's original llvm-dev post where he describes the issues in even more detail.

We should back-port those fixes into DXC.

Start with include/llvm/ADT/SparseBitVector.h, include/llvm/ADT/ilist.h, and include/llvm/ADT/ilist_node.h, then compare the referenced upstream LLVM ilist changes with the current DXC copies. Use unittests/ADT/SparseBitVectorTest.cpp under an UBSAN build to verify the downcast report is gone and existing SparseBitVector behavior remains covered.