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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

That's how API reference looks with this patch. I specifically picked small groups to showcase member grouping.
![image](https://github.com/llvm/llvm-project/assets/12883766/620d2da6-233c-436d-9ed7-a60b2192d1bb)
Existing API reference can be found here: https://clang.llvm.org/doxygen/classclang_1_1Sema.html

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