Fix the Visual Studio 2015 build of the recovery code and its tests

- parser::report_error() calls only sax->parse_error(). For a SAX parser
  whose parse_error() is static, as in unit-disabled_exceptions.cpp,
  Visual Studio 2015 then reports the parameter as unreferenced (C4100),
  which /WX turns into an error.
- The variadic concatenated() helper in unit-sax_parse.cpp made Visual
  Studio 2015 fail with an internal compiler error (C1001). It now takes
  an initializer list of byte vectors.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -20646,6 +20646,7 @@ class parser
     JSON_INTERNAL_ALWAYS_INLINE
     std::false_type report_error(SAX* sax, const Exception& ex, std::false_type /*allow_recovery*/)
     {
+        static_cast<void>(sax); // MSVC 2015 does not count calling a static parse_error() as using it
         error_reported = true;
         static_cast<void>(sax->parse_error(m_lexer.get_position(), m_lexer.get_token_string(), ex));
         return {};
@@ -20658,6 +20659,7 @@ class parser
     template<typename SAX, typename Exception>
     bool report_error(SAX* sax, const Exception& ex, std::true_type /*allow_recovery*/)
     {
+        static_cast<void>(sax); // MSVC 2015 does not count calling a static parse_error() as using it
         const std::size_t position = m_lexer.get_position().chars_read_total;
         if (error_reported && position == last_error_position && last_token == last_error_token)
         {