[clang] [Headers][X86] Allow SLLDQ/SRLDQ byte shift intrinsics to be used in constexpr (PR #164166)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I don't think I have an immediate problem with the implementation (at least not the parts in `InterpBuiltin.cpp`, good job), but the two added functions seem almost identical - maybe unifying the implementation and doing different things in the loop depending on the builtin ID would work? I'm not sure right now if there are other functions like this and id a lambda-based implementation would be better. Maybe @RKSimon knows.

https://github.com/llvm/llvm-project/pull/164166