[clang] [clang] implement current direction of CWG2765 for string literal comparisons in constant evaluation (PR #109208)

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

We need to figure out what we want to do about modules, but I think I've addressed the rest of the comments.

https://github.com/llvm/llvm-project/pull/109208