[clang] [clang] Mark `__builtin_convertvector` and `__builtin_shufflevector` as `constexpr`. (PR #112129)

[via cfe-commits]

c8ef wrote:

I believe the difference lies in the AST construction. Most built-in functions generate a `CallExpr` AST node, which requires the constant evaluator to check if the function is built-in. However, in this case, Clang emits a `ConvertVector` node directly. Therefore, even if we don't mark it as `constexpr`, we can still evaluate it.

https://github.com/llvm/llvm-project/pull/112129