Replaced C++2a with C++20 in clang-tools-extra

Reviewers: hlopko, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: wuzish, aaron.ballman, nemanjai, kbarton, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81087