Clang UBSAN currently complains that the char * to input_buffer_adapter
is a nullptr.

Turns out it is actually required to accept nullptr, see for example
line 415 in input_adapters.hpp

  ...
  // the address of first cannot be used: use nullptr
  ia = std::make_shared<input_buffer_adapter>(nullptr, len);
  ....

Therefore we have to handle it gracefully here. We now also ignore the
length parameter l if b is a nullptr.

- Switch to clang-7
- Adapt PATH so that llvm-symbolizer can be found for useful stacktraces
- Adapt compile flags
  "-O0" ensures much faster compile times
  "-fno-sanitize-recover=all
  -fsanitize-recover=unsigned-integer-overflow" this fails the build on
  all issues except unsigned integer overflows. Not failing in this case
  is required in combination with the sanitizer suppression file as only
  recoverable errors can be suppressed.

The UBSAN suppression file ignores errors from stl_bvector.h (which
holds std::vector<bool>).

Clang reports that error as

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_bvector.h:158:20 in

      Start 34: test-deserialization_all
28/88 Test #71: test-testsuites_default .............***Failed    0.32 sec
/usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_bvector.h:158:20: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned int'
    #0 0x628f72 in std::_Bit_iterator_base::_M_bump_down() /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_bvector.h:158:20
    #1 0x628d16 in std::_Bit_iterator::operator--() /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_bvector.h:251:7
    #2 0x634aac in std::vector<bool, std::allocator<bool> >::pop_back() /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_bvector.h:1010:7
    #3 0x61eff0 in bool nlohmann::detail::parser<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer> >::sax_parse_internal<nlohmann::detail::json_sax_dom_parser<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer> > >(nlohmann::detail::json_sax_dom_parser<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer> >*) /home/firma/devel/json/include/nlohmann/detail/input/parser.hpp:439:28
    #4 0x604864 in nlohmann::detail::parser<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer> >::parse(bool, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer>&) /home/firma/devel/json/include/nlohmann/detail/input/parser.hpp:116:13
    #5 0x5f8079 in nlohmann::operator>>(std::istream&, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer>&) /home/firma/devel/json/include/nlohmann/json.hpp:6356:42
    #6 0x5e1d92 in _DOCTEST_ANON_FUNC_21() /home/firma/devel/json/test/src/unit-testsuites.cpp:343:9
    #7 0x7207fe in doctest::Context::run() /home/firma/devel/json/test/thirdparty/doctest/doctest.h:5938:21
    #8 0x72681a in main /home/firma/devel/json/test/thirdparty/doctest/doctest.h:6016:71
    #9 0x7f75d22362e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #10 0x4c28b9 in _start (/home/firma/devel/json/build/test/test-testsuites+0x4c28b9)

The pop_back() in parser.hpp

      assert(not states.empty());
  ->  states.pop_back();

triggers the UBSAN report. But the assertion above ensure that we only
call pop_back() on an non-empty vector, therefore this is a STL library
bug and thus must be ignored for us.

Add gcc 9 and compile with experimental C++20 support

Use GNUInstallDirs instead of hard-coded path.

Package Manager: MSYS2 (pacman)

Add a package installation instructions for MSYS2 software distro.

Fix json.hpp compilation issue with other typedefs with same name (Issue #1642)