[Clang][Frontend] Change help text for --offload-host-device

I believe the help text for the flag `--offload-host-device` is wrong. Currently, the help text says "Only compile for the offloading host." but the flag alias with `--cuda-compile-host-device` which
has the help text "Compile CUDA code for both host and device (default). Has no effect on non-CUDA compilations."

Fixes https://github.com/llvm/llvm-project/issues/64243

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D156641