[clang][Driver][SPIR-V] Allow linking IR using llvm-link (#169572)

SPIR-V does not have a production-grade linker so it is often necessary
to stay in LLVM-IR as long as possible and only convert to SPIR-V at the
very end.

As such, it is common that we want to create a BC library that is linked
into a user program (motivating example here is the OpenMP device RTL).
We only convert to SPIR-V at the very end when we have a fully linked
program.

Other targets can achieve a similar goal by using LTO to get a linked-BC
file, but the SPIR-V linker does not support LTO, so we have no way to
do it with `clang`. We can do it with `llvm-link` directly, but my
understanding is that it is not intended to be directly called in
production workflows and does not fit well into code that supports
multiple targets.

Extend the existing behavior for `HIP` that allows `-emit-llvm` without
`-c`.

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Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>