[clang] [clang][NFC] Refactor Builtins.def to be a tablegen file (PR #68324)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,333 @@
+//=- ClangDiagnosticsEmitter.cpp - Generate Clang diagnostics tables -*- C++ -*-
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AaronBallman wrote:

```suggestion
//=- ClangBuiltinsEmitter.cpp - Generate Clang  builtins tables -*- C++ -*-
```
May need to adjust formatting.

https://github.com/llvm/llvm-project/pull/68324