Upstream actually expects the use of particular commits of spirv-tools and spirv-headers that are provided in the known_good.json file.
However, if we went that route, we'd have to vendor the resulting spirv-tools libs from this glslang build and they would be incompatible with system libs, resulting in us having to ship both. Instead, it is recommend to just wait until upstream releases a compatible version of spirv-tools and spirv-header, then updates those packages and only then building glslang against that.

Silent ABI breakages

Careful when upgrading this package! It usually breaks ABI without bumping soname.
It is recommended to rebuild reverse dependencies on upgrade out of precaution (e.g. https://archlinux.org/todo/glslang-1510-rebuild/).