[NVPTX] Fix NVPTX output name in the driver with `-save-temps`

Summary:
Currently, OpenMP and direct compilation uses an NVPTX toolchain to
directly invoke the CUDA tools from Clang to do the assembling and
linking of NVPTX codes. This breaks under `-save-temps` because of a
workaround. The `nvlink` linker does not accept `.o` files, so we need
to be selective when we output these. The previous logic keyed off of
presense in the temp files and wasn't a great solution. Change this to
just query the input args for `-c` to see if we stop at the assembler.

Fixes https://github.com/llvm/llvm-project/issues/60767