[clang] [Clang][X86] Add avx512 __builtin_ia32_select* constexpr handling (PR #152705)

[Timm Baeder via cfe-commits]

tbaederr wrote:

This introduced an unused variable:
```
/home/tbaeder/code/llvm-project/clang/lib/AST/ExprConstant.cpp:11818:14: warning: unused variable 'DestEltTy' [-Wunused-variable]
 11818 |     QualType DestEltTy = E->getType()->castAs<VectorType>()->getElementType();
       |              ^~~~~~~~~

```

https://github.com/llvm/llvm-project/pull/152705