message: Fix a "memset clearing an object of non-trivial type" warning in DWARFFormValue
  This is diagnosed by gcc-8. The ValueType struct already has a default
  constructor which performs zero-initialization, so we can just call that
  instead of using memset.
diff: @@ -115,7 +115,7 @@ DWARFFormValue::DWARFFormValue(const DWARFUnit *cu, dw_form_t form)
  void DWARFFormValue::Clear() {
  m_cu = nullptr;
  m_form = 0;
  - memset(&m_value, 0, sizeof(m_value));
  + m_value = ValueTypeTag();
  }
  bool DWARFFormValue::ExtractValue(const DWARFDataExtractor &data,
  