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

[Andy Kaylor via cfe-commits]

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+# ClangIR ABI Lowering - Design Document
+
+## 1. Introduction
+
+This design describes calling convention lowering that **builds on the GSoC ABI
+Lowering Library** (PR #140112): we use its `abi::Type*` and target ABI logic
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andykaylor wrote:

Why is "builds on the GSoC ABI Lowering Library" in bold here?

We probably don't want to refer to the GSoC project or the PR directly here. We should just refer to the LLVM ABI Lowering Library (or whatever it is going to be called).

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