[llvm] [LLVMABI] Implement the ABI Typesystem (PR #158329)

[Nikita Popov via llvm-commits]

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nikic wrote:

Should add the license header here:
```
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
```

https://github.com/llvm/llvm-project/pull/158329