This PR implements the first part of the LLVM ABI lowering library,
proposed in [this
RFC](https://discourse.llvm.org/t/rfc-an-abi-lowering-library-for-llvm/84495).
It is split out of https://github.com/llvm/llvm-project/pull/140112,
which demonstrates how this is going to be used.
The ABI type system is intended to represent all the type information
that is necessary to make call lowering decisions. As such, it contains
less information than Clang QualTypes, but more information than LLVM IR
types. The current type system has enough information to implement the
x86_64 SysV ABI, but some extensions will likely be needed in the future
for other targets (e.g. unadjusted alignment).
The type system expects layout information (like size, offset and
alignment) to already be computed by the frontend.
The types are constructed using TypeBuilder, which uses a
BumpPtrAllocator. The types themselves are not uniqued -- instead we
cache the QualType -> ABI type translation (in future patches).
c2c787c16f