[Migrated] spirv-builder could show warnings from building shaders.

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/880
Old labels: t: enhancement
Originally creatd by eddyb on 2022-07-04T06:48:36Z

Cargo build scripts have the ability to produce output through Cargo via cargo:warning=MESSAGE even when successful.

We should probably make spirv-builder transform stderr lines from its nested cargo build (of the shaders) into cargo:warning, but we may need to use the JSON output mode to be able to tell apart Cargo output from rustc output (as we presumably don't want to dump Compiling ... messages as "warnings").

We already pass --message-format=json-render-diagnostics to the nested cargo build though (to parse out "artifact notifications"), so it should be pretty easy to further rely on the JSON output.

Start at spirv-builder's handling of the nested cargo build, which already uses --message-format=json-render-diagnostics to parse artifact notifications. Determine how Cargo warning messages are represented in that output, then ensure relevant shader-build warnings reach the outer build as cargo:warning lines without converting ordinary Cargo or rustc output into warnings.