[clang][ExprConst] Try to minimize emitting unneeded diagnostics

[tbaederr]

There are a few places in clang where we pass a list of diagnostic notes to the evaluate* functions, which so some extra work when that list is passed (e.g. attaching call stacks to the diagnostics). We then sometimes later don't use the notes list at all, or we only use it to check if any notes have been emitted.

This patch adds two new flags to EvalStatus to check if any diagnostics have been emitted (right now one of the would suffice though, I guess), which replaces the Notes.empty() use case.

It also changes the signature of EvaluateAsInitializer to pass an EvalStatus object instead, so we don't have to pass a notes list but can still examine the resulting flags.

[tbaederr]

This is good:
https://llvm-compile-time-tracker.com/compare.php?from=efa7385831503b38b45f8b4eca3e21ba7a261097&to=d17e469d8066482a00c0b647a0044a58c710bfc1&stat=instructions:u

But it breaks the __builtin_constant_p handling 🙄