Add ABITypeMapper and ABIRewriteContext as the dialect-agnostic
bridge between MLIR dialects and the LLVM ABI Lowering Library.
ABITypeMapper maps MLIR built-in types (integer, float, vector,
index, memref) to abi::Type* using DataLayout for sizes and
alignment. Dialect-specific types fall back to integer mapping
via DataLayoutTypeInterface.
ABIRewriteContext defines the abstract interface that each dialect
(CIR, FIR) implements to rewrite function definitions and call
sites after ABI classification. See the CIR ABI lowering design
document (clang/docs/ClangIRABILowering.md, Section 4) for the
architectural context.
Unit tests for both components (18 test cases).
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