[llvm] [SPIR-V] Skip available_externally functions in module analysis (PR #200002)

[Dmitry Sidorov via llvm-commits]

https://github.com/MrSidims requested changes to this pull request.

We should let `available_externally` functions and kernels in SPIR-V module. Whether to discard them or not must be decided by the linker or SPIR-V consumer.

I acknowledge, that currently these is no legal way to preserve `available_externally` in SPIR-V by SPIR-V backend, but there will be another mechanism. 

https://github.com/llvm/llvm-project/pull/200002