[CIR] Add MLIR ABI Lowering design document
Design document for MLIR dialect-agnostic calling convention
lowering that builds on the LLVM ABI Lowering Library
(llvm/lib/ABI/) as the single source of truth for ABI
classification. Dialects use the library via an adapter layer:
ABITypeMapper maps dialect types to abi::Type*, the library
classifies arguments and returns, and a dialect-specific
ABIRewriteContext applies the decisions back to IR operations.
Targets x86_64 and AArch64, with parity against Classic Clang
CodeGen validated through differential testing.
