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

[John McCall via llvm-commits]

rjmccall wrote:

I appreciate the elegance of having a type system that eliminates differences that don't really matter at the ABI level, but having to allocate a bunch of memory to translate types to a new representation that's used for a few cycles while we call this portable library seems like a pretty high amount of overhead.

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