[ci] Switch back to c++2a

Okay I guess we can be bothered to fix things up for next version of C++.

Reverts meat of c0b3b9b258

.circleci/config.yml

@@ -128,7 +128,7 @@ jobs:
       # msan, needs --force-fallback-for=glib,freetype2 also which doesn't work yet but runs fuzzer cases at least
       - run: rm -rf build && meson build --default-library=static -Db_sanitize=memory --buildtype=debugoptimized --wrap-mode=nodownload -Dauto_features=disabled -Dtests=enabled -Dexperimental_api=true
       - run: ninja -Cbuild -j8 && meson test -Cbuild --print-errorlogs | asan_symbolize | c++filt
-      - run: clang -c src/harfbuzz.cc src/hb-subset*.cc -DHB_NO_MT -Werror -std=c++17
+      - run: clang -c src/harfbuzz.cc src/hb-subset*.cc -DHB_NO_MT -Werror -std=c++2a
 
   crossbuild-win32:
     executor: win32-executor