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

[Andy Kaylor via cfe-commits]

================
@@ -0,0 +1,602 @@
+# 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
+and add an MLIR integration layer (MLIRTypeMapper, ABI lowering pass, and
+dialect rewriters).  The framework relies on the LLVM ABI library in
+`llvm/lib/ABI/` as the single source of truth for ABI classification; MLIR
----------------
andykaylor wrote:

```suggestion
`llvm/lib/ABI/` as the single source of truth for ABI classification. MLIR
```

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