[OpenCL] Do not guard vload/store_half builtins

The vload*_half* and vstore*_half* builtins do not require the
cl_khr_fp16 extension: pointers to `half` can be declared without the
extension and the _half variants of vload and vstore should be
available without the extension.

This aligns the guards for these builtins for
`-fdeclare-opencl-builtins` with `opencl-c.h`.

Fixes https://github.com/llvm/llvm-project/issues/55275

Differential Revision: https://reviews.llvm.org/D125401