[llvm] [LLVMABI] Implement the ABI Typesystem (PR #158329)

[John McCall via llvm-commits]

rjmccall wrote:

Okay, if that's total instructions *retired* rather than *emitted*, that's an interesting compile-time measurement, but it's also one we wouldn't expect to reflect the performance impact of significant changes in memory use patterns. I understand that wall-clock measurements are noisy on the level of 1-2%, but that means measuring them requires more attention, not that we should just absorb arbitrary accumulated regressions because they're all within the noise. IRGen is generally a pretty small contributor to the overall compile time, so a .6% increase in overall compile time actually means a very large relative increase in IRGen time.

https://github.com/llvm/llvm-project/pull/158329