Add ClangCL for MSVC

MS Visual Studio 2019 has builtin support for other compiler toolsets [1].

This commit adds support for compiling using LLVM/Clang 10 using Visual
Studio.

ClangCL pretends to be MSVC so the usual MSVC flags apply, see also [2].
For detecting if ClangCL is used, newer cmake verisons (>= 3.15) have
builtin support using CMAKE_CXX_COMPILER_FRONTEND_VARIANT [3], for older
ones a workaround is available using CMAKE_CXX_SIMULATE_ID [4].

[1]: https://devblogs.microsoft.com/cppblog/clang-llvm-support-in-visual-studio/
[2]: https://clang.llvm.org/docs/UsersManual.html#clang-cl
[3]: https://stackoverflow.com/a/10055571
[4]: https://github.com/microsoft/CMake/commit/4fe34b2d29ca67ea91b2545b223dad8abbcad282

Makefile: Always use SED variable

Add Xcode 12 CI

We need to have a proper sed, even on MacOSX. So let's use the variable
introduced in 191aa0fd (🔧 overworked maintaner targets,
2019-03-28) in more places.

Add ordered_json specialization with ordered object keys

Make library work with Clang on Windows

 Conflicts:
	include/nlohmann/detail/input/binary_reader.hpp
	include/nlohmann/detail/input/json_sax.hpp
	include/nlohmann/detail/input/lexer.hpp
	include/nlohmann/detail/input/parser.hpp
	include/nlohmann/detail/json_pointer.hpp
	include/nlohmann/detail/output/serializer.hpp
	include/nlohmann/json.hpp
	single_include/nlohmann/json.hpp

 Conflicts:
	single_include/nlohmann/json.hpp

Simple ordered_json that works on all supported compilers

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

Add pkg-config file

Make assert configurable via JSON_ASSERT

The meson builds install a pkg-config file, but the cmake builds don't.
This adds a pkg-config file to the cmake builds that is functionally
equivalent to the one generated from meson.

Fix regression from #2181