[clang] Add code completion for C++20 keywords. (PR #107982)

[via cfe-commits]

16bit-ykiko wrote:

> This change needs a release note. Please add an entry to `clang/docs/ReleaseNotes.rst` in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

I an not sure which section I should add in `clang/docs/ReleaseNotes.rst`?  There isn't a section related to code completion.

https://github.com/llvm/llvm-project/pull/107982