glslang shouldn't require that specialization constant ids must be unique

Pending the discussion in https://github.com/KhronosGroup/SPIRV-Registry/issues/357 we should consider removing the restriction on `constant_id` qualifiers that requires them to be unique. Neither the GLSL spec nor the `GL_KHR_vulkan_glsl` extension actually require uniqueness, so if it's allowed by SPIR-V it should probably be allowed by glslang as well.

Start by reading the linked SPIRV-Registry discussion and then trace where glslang validates that `constant_id` qualifiers are unique. Done means resolving the specification question and ensuring glslang's behavior matches the accepted SPIR-V and GLSL requirements.

Written by the indexing model from the issue text.