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

[Nikita Popov via llvm-commits]

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nikic wrote:

```suggestion
  LLVM_MARK_AS_BITMASK_ENUM(/* LargestValue = */ HasFlexibleArrayMember)
}
```
This one needs to go inside the enum declaration. You should also be able to drop the three `operator`s below.

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