objective-c++: add support for current supported standards in Apple Clang

Pulled from this list on Xcode 15 beta:

    $ clang -E -dM -xobjective-c++ -std=arglbargle -o - /dev/null
    error: invalid value 'arglbargle' in '-std=arglbargle'
    note: use 'c++98' or 'c++03' for 'ISO C++ 1998 with amendments' standard
    note: use 'gnu++98' or 'gnu++03' for 'ISO C++ 1998 with amendments and GNU extensions' standard
    note: use 'c++11' for 'ISO C++ 2011 with amendments' standard
    note: use 'gnu++11' for 'ISO C++ 2011 with amendments and GNU extensions' standard
    note: use 'c++14' for 'ISO C++ 2014 with amendments' standard
    note: use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
    note: use 'c++17' for 'ISO C++ 2017 with amendments' standard
    note: use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
    note: use 'c++20' for 'ISO C++ 2020 DIS' standard
    note: use 'gnu++20' for 'ISO C++ 2020 DIS with GNU extensions' standard
    note: use 'c++2b' for 'Working draft for ISO C++ 2023 DIS' standard
    note: use 'gnu++2b' for 'Working draft for ISO C++ 2023 DIS with GNU extensions' standard

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

mesonbuild/compilers/objcpp.py

@@ -96,7 +96,9 @@ class ClangObjCPPCompiler(ClangCompiler, ObjCPPCompiler):
         opts.update({
             OptionKey('std', machine=self.for_machine, lang='cpp'): coredata.UserComboOption(
                 'C++ language standard to use',
-                ['none', 'c++98', 'c++11', 'c++14', 'c++17', 'gnu++98', 'gnu++11', 'gnu++14', 'gnu++17'],
+                ['none', 'c++98', 'c++11', 'c++14', 'c++17', 'c++20', 'c++2b',
+                 'gnu++98', 'gnu++11', 'gnu++14', 'gnu++17', 'gnu++20',
+                 'gnu++2b'],
                 'none',
             )
         })