[Clang] Accept deducing this as an extension before C++23

[philnik777]

Clang doesn't allow the use of deducing this before C++23, but GCC accepts it in all standards modes. It would be great if Clang could support it as a conforming extension as well, since it'd open the door for some major simplifications in libc++.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nikolas Klauser (philnik777)

Clang doesn't allow the use of deducing this before C++23, but GCC accepts it in all standards modes. It would be great if Clang could support it as a conforming extension as well, since it'd open the door for some major simplifications in libc++.