[clang] [clang][NFC] Regroup declarations in `Sema` (PR #82217)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@erichkeane 

> That said, waiting until after 18 is perhaps a good diea.

Resolving merge conflicts that will arise in the meantime is not going to be trivial, but should be doable in a reasonable time. So I'm willing to wait.

> I MIGHT suggest private followed by public? It is a not-uncommon pattern I've seen to have a private 'helper' class(or data member) defined inline, that is then used by inline-defined public functions.

I don't mind either way, as long as we don't inserting a couple of private members in the middle of public section.

> I wouldn't mind some sort of 'static_assert' to ensure that this doesn't accidentally increase the size of Sema

Checking on Linux, `sizeof(Sema)` is 18824 bytes at moment. This patch increases that by 24 bytes, to `18848`. I don't deem it significant enough to care for an object this big, but I can claw it back if this is considered important.

> or cause some sort of pessimization for layout. I realize we're not particularly concerned about the size, but I could imagine goofy things going on.

If there was an intent to put some data members first to improve cache hits, I think it has been lost by this point. Our first non-static data members are `OpenCLOptions OpenCLFeatures` and `FPOptions CurFPFeatures` at the moment. Since this patch puts generic Sema stuff (`Sema.cpp`) first, opportunity to put widely used data members first is still there. As I mentioned in the description, follow-up patches that improve ordering in each particular section are expected.

https://github.com/llvm/llvm-project/pull/82217