Isn’t glslang an official GLSL to SPIR-V compiler?
Khronos is hoping glslang will be the standard, although the community is welcome to build on that. It’s an open-source project, and it implements the new version of GLSL that supports the Vulkan features.
The Khronos Github includes a gslang folder. It’s the official reference Khronos validation layer to validate that the GLSL you’re using conforms to the specification. You can translate or modify it for Vulkan GLSL into SPIR-V, then send that into the drivers for the different platform.
Anyone is welcome to contribute to the project. The lead author, John Kessenich, is an active member of the working group, and he is also the editor of the GLSL specification. Khronos endorses glslang as an ESSL/GLSL validator. Khronos does not guarantee that it is bug free, but does guarantee to expend effort to fix it if there are issues. You are certainly allowed to create your own GLSL/SPIR-V compiler, but Khronos would rather fix any bugs in glslang.