GL_HUAWEI_subpass_shading support

[Karpov-Ruslan]

Can you add support for GL_HUAWEI_subpass_shading and corresponding SPV_HUAWEI_subpass_shading?

Here we need a translator from GLSL to SPIR-V that supports GL_HUAWEI_subpass_shading features.

[arcady-lunarg]

In general it has been up to the extension promoters to add support for their extensions to glslang, which apparently Huawei did not do in this case.