[c++] DynamicParser may not emit OmittedField for compile-time schema

[ara-ayvazyan]

Under some conditions, bond::DynamicParser may not emit OmittedField for a compile-time schema. For the most transforms that is implemented as a no-op. However, bond::Merge relies on that in order to take data from a struct.

/*
    struct ABC
    {
        0: int8 a;
        1: int8 b;
        2: int8 c;
    };
    struct B view_of ABC
    {
        b;
    };
*/
bond::CompactBinaryReader<bond::InputBuffer> abc_payload{ {} };
{
    ABC abc;
    abc.a = 'a';
    // Leave ABC::b with default value, such that it will be skipped in the payload.
    abc.c = 'c';

    bond::OutputBuffer output;
    bond::CompactBinaryWriter<bond::OutputBuffer> writer{ output };
    bond::Serialize(abc, writer);
    abc_payload = { output.GetBuffer() };
}
bond::CompactBinaryReader<bond::InputBuffer> abc_merged_b_payload{ {} };
{
    B b;
    b.b = 'b';

    bond::OutputBuffer output;
    bond::CompactBinaryWriter<bond::OutputBuffer> writer{ output };
    bond::Merge(b, abc_payload, writer);
    abc_merged_b_payload = { output.GetBuffer() };
}
ABC abc;
bond::Deserialize(abc_merged_b_payload, abc);
assert(abc.a == 'a');
assert(abc.b == 'b');   // Fails, since OmittedField is not called for B::b
assert(abc.c == 'c');
B b;
bond::Deserialize(abc_merged_b_payload, b);
assert(b.b == 'b');     // Fails, since OmittedField is not called for B::b