[clang] [Clang][NFC] Remove unused debug macro (PR #136400)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Actually it's used :(
```cpp
/var/lib/buildkite-agent/builds/linux-56-59b8f5d88-5b8n6-1/llvm-project/github-pull-requests/clang/lib/AST/ExprConstant.cpp:11780:3: error: use of undeclared identifier 'DEBUG_TYPE'
  | LLVM_DEBUG(llvm::dbgs() << "The number of elements to initialize: "
  |  

```

https://github.com/llvm/llvm-project/pull/136400