[clang] [CIR] Add x86_64 aggregate calling-convention lowering (PR #210528)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm ok with this, but can't claim to understand this/the direction enough to do the final approval.  Andy should at least have a chance to look at it.

https://github.com/llvm/llvm-project/pull/210528