HLSL: Add support for printf().

Translate printf() to what GL_EXT_debug_printf has done. HLSL could
define non-constant string variable and we don't have such features
in SPIR-V, so just support constant string variable.

glslang/Include/Types.h

@@ -1986,6 +1986,7 @@ public:
         case EbtAccStruct:         return "accelerationStructureNV";
         case EbtRayQuery:          return "rayQueryEXT";
         case EbtReference:         return "reference";
+        case EbtString:            return "string";
 #endif
         default:                   return "unknown type";
         }