[clang] [NFC][clang] Split clang/lib/CodeGen/CGBuiltin.cpp into target-specific files (PR #132252)

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github-actions[bot] wrote:

<!--LLVM CODE FORMAT COMMENT: {undef deprecator}-->


:warning: undef deprecator found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

``````````bash
git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 9b1f905b483a2b42e4681bbef42b2641f8ccf5ad 7b7ad22cf4cd44b45c655558f8653db9e5346fd5 clang/lib/CodeGen/BuiltinTargets/AArch64.cpp clang/lib/CodeGen/BuiltinTargets/AMDGPU.cpp clang/lib/CodeGen/BuiltinTargets/Hexagon.cpp clang/lib/CodeGen/BuiltinTargets/NVPTX.cpp clang/lib/CodeGen/BuiltinTargets/PPC.cpp clang/lib/CodeGen/BuiltinTargets/RISCV.cpp clang/lib/CodeGen/BuiltinTargets/SPIR.cpp clang/lib/CodeGen/BuiltinTargets/SystemZ.cpp clang/lib/CodeGen/BuiltinTargets/WebAssembly.cpp clang/lib/CodeGen/BuiltinTargets/X86.cpp clang/lib/CodeGen/CGBuiltin.h clang/lib/CodeGen/CGBuiltin.cpp
``````````

</details>


The following files introduce new uses of undef:
 - clang/lib/CodeGen/BuiltinTargets/AArch64.cpp
 - clang/lib/CodeGen/BuiltinTargets/X86.cpp

[Undef](https://llvm.org/docs/LangRef.html#undefined-values) is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields `undef`. You should use `poison` values for placeholders instead.

In tests, avoid using `undef` and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:
```llvm
define void @fn() {
  ...
  br i1 undef, ...
}
```

Please use the following instead:
```llvm
define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}
```

Please refer to the [Undefined Behavior Manual](https://llvm.org/docs/UndefinedBehavior.html) for more information.



https://github.com/llvm/llvm-project/pull/132252