This PR implements the System V X86-64 ABI for the LLVM ABI Library
prototyped in https://github.com/llvm/llvm-project/pull/140112, and
wires it into clang's Codegen.
`X86_64TargetInfo` is a direct parallel to
`clang::CodeGen::X86_64ABIInfo`, but operates entirely on the
`llvm::abi` type system.
The AMD64 ABI classification spec, argument/return lowering , and the
supporting helpers are all reproduced against `llvm::abi::Type` and
FunctionInfo, keeping the ABI logic frontend-independent.
This, similar to the [BPF ABI implementaiton
](https://github.com/llvm/llvm-project/pull/194031/changes)is gated
under the `-fexperimental-abi-lowering` flag.
Narayan
cfa5d6f59d