Validate vertex entry point returns the position built-in

[teoxoy]

fixes gfx-rs/wgpu#3550

[jimblandy]

I was wondering if it was right to enforce this on all modules, not just those derived from WGSL. But:

• GLSL says the vertex position is undefined if the vertex shader doesn't assign to gl_Position, so it seems like any vertex shader that doesn't do so should probably be flagged by Naga.

• I can't find anything in the Vulkan or SPIR-V specifications that justifies enforcing this rule on SPIR-V input, but I also don't see the harm.

So, I guess we might as well go with this and see if anyone complains. Thanks for updating all the tests.

I think there's a better way to do the check.

[jimblandy]

Thanks!