[CIR] Correct union lowering behavior re-padding (#216349)
Most of the code that CIR lowers to LLVM counts on the fact that our
alignnments are correct/calculated in LLVM to get our layout correctly.
This works for the most part, and unions have the storage type of the
'highest' alignment type.
However, when creating a constant, we have to convert the type of the
union to have a 'storage' type that matches the data being inserted (not
the union's storage type!). The result was that if we had a storage type
where the alignment was smaller than the actual storage type, LLVM would
mis-calculate the padding.
This patch adds the padding explicitly when we make that conversion to
get the alignment set up correctly.
Note: there is one mild IR-equivilency-regression to this patch. There
isn't really a great way to tell the difference between a
union-tail-padding needing zero-init vs undef-init in this case. This
patch chooses to make it always zero-init, which is harmless. While it
MIGHT suppress some optimizations (facts not in evidence?), it seems
like something we can figure out later if necessary.
