"No OpEntryPoint instruction was found" for public function

Expected Behaviour

Non-public submodules should be supported, but instead I'm getting this:

     Compiling ab-proof-of-space-gpu v0.1.0 (ab-proof-of-space-gpu)
  warning: function `main_cs` is never used
    --> src/shaders.rs:10:8
     |
  10 | pub fn main_cs(
     |        ^^^^^^^
     |
     = note: `#[warn(dead_code)]` on by default

  error: No OpEntryPoint instruction was found. This is only allowed if the Linkage capability is being used.
    |
    = note: module `target/debug/build/ab-proof-of-space-gpu-dc89fe269d0c81ad/out/spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

  warning: an unknown error occurred
    |
    = note: spirv-opt failed, leaving as unoptimized
    = note: module `target/debug/build/ab-proof-of-space-gpu-dc89fe269d0c81ad/out/spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

  error: error:0:0 - No OpEntryPoint instruction was found. This is only allowed if the Linkage capability is being used.
    |
    = note: spirv-val failed
    = note: module `target/debug/build/ab-proof-of-space-gpu-dc89fe269d0c81ad/out/spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

Slapping #[unsafe(no_mangle)] is sufficient to bypass this error, making submodule publicly reachable too.

I think #[spirv(..)] macro should be able to do something about it automatically to ensure it is exposed properly.

Kind of related to https://github.com/Rust-GPU/rust-gpu/issues/156

Example & Steps To Reproduce

1. Have a shader in submodule that is not publicly reachable

Reproduce the failure with a shader function in a non-public submodule, using the reported #[spirv(..)] macro and comparing it with the #[unsafe(no_mangle)] workaround. Inspect the macro's handling of public functions and the generated SPIR-V entry points; done means the non-public submodule case validates without requiring the manual attribute.