[clang-tools-extra] [libcxx] Allow string to use SSO in constant evaluation. (PR #66576)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

I think this is fine but I agree we should have a compat warning for this and I would like to see more testing as well.

https://github.com/llvm/llvm-project/pull/66576