[CIR] Accept unions in x86_64 calling-convention lowering (#214129) Passing a union to a function does not work. The x86_64 bridge rejects every one, so the pass fails on any signature naming a union. Additionally, indirect arguments have their byval and sret alignment wrong, because `mapCIRType` asks DataLayout for it, and DataLayout only sees a record's members, never `__attribute__((aligned(N)))`. Unions now go through the ABI library's union type, which puts every member at offset zero and sizes each eightbyte from the union rather than from a single member. The alignment comes from the record-layout metadata the AST already fills in, which fixes over-aligned structs too, since they share that lookup. Assisted-by: Cursor / claude-opus-5
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