[Clang] Allow __builtin_elementwise_fshl/fshr builtin intrinsics to be used in constexpr

[RKSimon]

Followup to #152555 - allow the new __builtin_elementwise_fshl/fshr builtin intrinsics to be used in constexpr

CC @ckoparkar

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Simon Pilgrim (RKSimon)

Followup to #152555 - allow the new __builtin_elementwise_fshl/fshr builtin intrinsics to be used in constexpr

CC @ckoparkar

[ckoparkar]

Working on it, thanks for creating the tracking issue.