[clang] [C++26] Stop diagnosing #embed as an extension in C++26 mode (P1967R14) (PR #233285)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

Following the discussion in the C++ WG, we can go ahead and merge this.
The bug fixes can delth with the other currently open PRs.

#224769 / CWG3013 need more discussion in the WG (we are not ready to make a call either way) - which again, is not a blocker.

https://github.com/llvm/llvm-project/pull/233285