[clang] [clang] Constant-evaluate format strings as last resort (PR #135864)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@apple-fcloutier I feel like this PR can be made easier to review by separating the improvements to string constant evaluation from the improvements to the format string checking, as the former introduces a number of new interfaces and the operations required to provide those APIs, which is an independent problem from the format string checking

https://github.com/llvm/llvm-project/pull/135864