[llvm] change contents of ScalarEvolution from private to protected (PR #83052)

[Joshua Ferguson via llvm-commits]

skewballfox wrote:

sorry if ask any dumb questions, my c++ is a bit rusty. Would it be acceptable to make `ExitLimitCache` public? currently `MustExitScalarEvolution` depends on it, and that I think needs to be public or protected for the stuff with Enzyme

https://github.com/llvm/llvm-project/pull/83052