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

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apple-fcloutier wrote:

@AaronBallman, @cor3ntin, what do you think? Split this in two, [like Oliver suggests](https://github.com/llvm/llvm-project/pull/135864#issuecomment-2879466843)?

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