[clang] [CIR] Correct union lowering behavior re-padding (PR #216349)

[Andy Kaylor via cfe-commits]

================
@@ -416,28 +416,68 @@ static bool shouldPackFAMStruct(const mlir::DataLayout &dataLayout,
 // Additionally, the struct itself could contain a struct with a FAM or a union
 // that needed adjustment, so it recurses to check those.  If no such type has
 // been found/no adjustment needed, this returns the type unchanged.
+//
+// Additionally, a union having an active member of the not-the-largest
----------------
andykaylor wrote:

```suggestion
// Additionally, a union having an active member of not-the-largest
```
I'm not sure about this. It read awkwardly to me.

https://github.com/llvm/llvm-project/pull/216349