[OpenMP] Add kernel string attribute to kernel function

This patch adds a function attribute to the kernel function generated in
OpenMP offloading. We already create a `nvvm.annotations` metadata node
indicating the kernels present in the program. However, this created
some indirection when trying to identify if a specific function was an
entry. We add a single function attribute for each function now to
simplify this.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D118708