Add support for spirv1.6

Add command line support which enables the following behavior:

- Remap discard

  Map discard to DemoteToHelperInvocation for HLSL shaders. Map to
  OpTerminateInvocation for GLSL shaders.

- Decorate HelperInvocation with Volatile

- Use localSizeId for execution mode

  WorkGroupSize is deprecated in spirv1.6

Also update known goods to SPIRV 1.6

glslang/CInterface/glslang_c_interface.cpp

@@ -244,6 +244,8 @@ c_shader_target_language_version(glslang_target_language_version_t target_langua
         return glslang::EShTargetSpv_1_4;
     case GLSLANG_TARGET_SPV_1_5:
         return glslang::EShTargetSpv_1_5;
+    case GLSLANG_TARGET_SPV_1_6:
+        return glslang::EShTargetSpv_1_6;
     default:
         break;
     }