[C++] Add meta attribute PRESENCE to generated codecs. Issue #483.

Author: mjpt777

sbe-tool/src/main/cpp/sbe/sbe.h

@@ -94,7 +94,8 @@ enum Attribute
 {
     EPOCH,
     TIME_UNIT,
-    SEMANTIC_TYPE
+    SEMANTIC_TYPE,
+    PRESENCE
 };
 
 }

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -1552,13 +1552,15 @@ private static void generateFieldMetaAttributeMethod(
             indent + "            case MetaAttribute::EPOCH: return \"%s\";\n" +
             indent + "            case MetaAttribute::TIME_UNIT: return \"%s\";\n" +
             indent + "            case MetaAttribute::SEMANTIC_TYPE: return \"%s\";\n" +
+            indent + "            case MetaAttribute::PRESENCE: return \"%s\";\n" +
             indent + "        }\n\n" +
             indent + "        return \"\";\n" +
             indent + "    }\n",
             token.name(),
             epoch,
             timeUnit,
-            semanticType));
+            semanticType,
+            encoding.presence().toString().toLowerCase()));
     }
 
     private static CharSequence generateEnumFieldNotPresentCondition(