[NVPTX] Add initial support for '.alias' in PTX

[NVPTX] Add initial support for '.alias' in PTX

This patch adds initial support for using aliases when targeting PTX. We
perform a pretty strict conversion from the globals referenced to the
expected output. as described in the PTX documentation at
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#kernel-and-function-directives-alias

These cannot currently be used due to a bug in the `nvlink`
implementation that causes aliases to pruned functions to crash the
linker.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D155211