IR "imports" section

[Napokue]

For WGSL, it is possible to use the "GLSL.std.450" import, as can be seen here in the spec. WGSL constraints this to only this import.

For instance, the SPIR-V backend now always includes this import statically, even though it isn't present in the WGSL. This causes an extra instruction, but it would be nice to keep it clean.

To continue on the previous point: as the import is always statically included, other front-ends are also limited by the WGSL constraint of only having the "GLSL.std.450" import available.

I think it would be a good idea to have a representation of "imports" in the IR so all the back-ends only have to loop through it and include it.

[kvark]

Note that WebGPU group figured out to postpone use statement and imports until after MVP.
For now, the standard library is going to be in the root scope.

[teoxoy]

Closing as outdated. It might be worth writing out the "GLSL.std.450" extension lazily though. Opened gfx-rs/wgpu#4504.