Fix flatc code gen.

Mutable functions should return a non-const reference to the member variable.

Change-Id: I833077d2c9a38a5d71868e43aca6b250d79b6625

src/idl_gen_cpp.cpp

@@ -1928,6 +1928,8 @@ class CppGenerator : public BaseGenerator {
       code_ += "  }";
 
       if (parser_.opts.mutable_buffer) {
+        auto mut_field_type = GenTypeGet(field.value.type, " ", "", " &", true);
+        code_.SetValue("FIELD_TYPE", mut_field_type);
         if (is_scalar) {
           code_.SetValue("ARG", GenTypeBasic(field.value.type, true));
           code_.SetValue("FIELD_VALUE",

tests/monster_test_generated.h

@@ -243,7 +243,7 @@ MANUALLY_ALIGNED_STRUCT(16) Vec3 FLATBUFFERS_FINAL_CLASS {
   const Test &test3() const {
     return test3_;
   }
-  const Test &mutable_test3() {
+  Test &mutable_test3() {
     return test3_;
   }
 };