[clang] [CIR][ABI][NFC] Add x86_64 ABI parity tests (PR #191259)

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adams381 wrote:

You're right — these document known ABI parity gaps without a mechanism to track when they're fixed.  The gaps (sret, byval, struct coercion) are addressed by the CIRABIRewriteContext work in #192119/#192124, which currently only have unit tests.  I'll fold these test cases into those PRs as integration tests (with CHECK lines updated to show parity) and close this one.

https://github.com/llvm/llvm-project/pull/191259