[clang] [clang] make it possible to disable c++26 type aware allocators (PR #138372)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Adding @AaronBallman and @cor3ntin as reviewers as you were the primary reviewers for the main PR, and I can't recall which of you requested I remove the flag and c++26 gating :D

https://github.com/llvm/llvm-project/pull/138372