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/MachineIndependent/localintermediate.h

@@ -398,6 +398,9 @@ public:
         case EShTargetSpv_1_5:
             processes.addProcess("target-env spirv1.5");
             break;
+        case EShTargetSpv_1_6:
+            processes.addProcess("target-env spirv1.6");
+            break;
         default:
             processes.addProcess("target-env spirvUnknown");
             break;