[HIP] Emit kernel symbol

Currently clang uses stub function to launch kernel. This is inconvenient
to interop with C++ programs since the stub function has different name
as kernel, which is required by ROCm debugger.

This patch emits a variable symbol which has the same name as the kernel
and uses it to register and launch the kernel. This allows C++ program to
launch a kernel by using the original kernel name.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D86376