[clang][AArch64] Correct return type of Neon vqmovun intrinsics

Neon intrinsics vqmovunh_s16, vqmovuns_s32, vqmovund_s64
should have unsigned return types.

See https://developer.arm.com/architectures/instruction-sets/simd-isas/neon/intrinsics?search=vqmovun

Fixes https://bugs.llvm.org/show_bug.cgi?id=46840

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D85118