[Clang] Add flags and lang option for C++2d (#203992)
This adds diagnostic groups, command-line options, a language option,
and various other things required to support `-std=c++2d`. I haven’t
touched e.g. clang-tidy or clang-format, as I’m not really familiar w/
either. This is only meant to add the most necessary parts to support
C++29 mode.
I grepped for `CPlusPlus26`/`C++26`/`C++2c` and updated all places I
could find where we need to handle every language mode in some way or
another.
This patch is roughly based on b763d6a4ed4650c74c6846d743156468563b0e31.
