[clang] [ClangIR] Add ABI Lowering Design Document (PR #178326)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

The biggest thing that jumps out at me is that you desperately need to replace "GSoC" with some more descriptive, but still brief, name throughout the document. There should be no references to "GSoC".

Otherwise, this is looking really good.

https://github.com/llvm/llvm-project/pull/178326