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

[via cfe-commits]

cor3ntin wrote:

I'm fairly opposed to adding this flag before Sofia - My assumption is that this is a C++26 feature (and one that is backward compatible) - and we should not add a flag for every standard feature. _if_ the committee fails to land this in Sofia, then we can add the flag before the 21 release

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