[clang] [CIR][NFC] Refactor calling convention lowering classification (PR #233639)

[Andy Kaylor via cfe-commits]

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andykaylor wrote:

I was trying to keep this change NFC, but we will definitely need better address space handling when https://github.com/llvm/llvm-project/pull/220197 merges. This whole function is a hot mess. Cleaning it up is one of my top priorities for this code after I get it refactored.

https://github.com/llvm/llvm-project/pull/233639