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

[via llvm-commits]

vortex73 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.

The prototype shows compile-time impact of [<0.1%](https://llvm-compile-time-tracker.com/compare.php?from=5886a276ed24c2cd936e602441365bb0bbe5ef40&to=a419e44cee5a9de7d3720133c3ad98b0174923e6&stat=instructions:u) at -O0, even without any performance optimization work. 

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