Add a WIP WebGPU environment. It disallows OpUndef

Add SPV_ENV_WEBGPU_0 for work-in-progress WebGPU.

val: Disallow OpUndef in WebGPU env

Silence unused variable warnings when !defined(SPIRV_EFFCE)

Limit visibility of validate_instruction.cpp's symbols
  Only InstructionPass needs to be visible so all other functions are put
  in an anonymous namespace inside the libspirv namespace.