Detect C++ language from compiler arguments

[ArthurVasseur]

This PR improves C++ language detection by analyzing compiler arguments.

Before my PR, the language was detected by checking if "++" existed in the compiler name. This approach is incorrect in certain cases.

cppast/src/libclang/libclang_parser.cpp

Lines 232 to 234 in 5329e37

	// If ++ exists within the compiler name (e.g. clang++, g++, etc), use C++
	std::string exe(clang_getCString(clang_CompileCommand_getArg(cmd, 0)));
	use_c_ = (exe.find("++", 0) == std::string::npos);

e.g my build utility xmake is only using clang not clang++ in the compile_commands.json and setting -std=c++20.