This PR introduces `ABIFunctionInfo` and surrounding utility helpers,
and is part of the set of breakout PRs to upstream the LLVM ABI lowering
library prototyped in https://github.com/llvm/llvm-project/pull/140112.
`ABIFunctionInfo` is directly analogous to `CGFunctionInfo` from Clang's
existing CodeGen pipeline, and represents an ABI lowered view of the
function signature, decoupled from both the Clang AST and LLVM IR.
`ABIArgInfo` encodes lowering decisions and currently supports
Direct,Extend,Indirect and Ignore which are required for our initial
goal of implementing x86-64 SysV and BPF, but this will change as the
library grows to represent more targets that need them.
This PR is a direct precursor to the implementation of `ABIInfo` in the
library as demonstrated in the PR linked above..
04785adec3