[clang][NFC] Regroup declarations in Sema

This patch regroups declarations in `Sema` based on the file they are implemented in (e.g. `SemaChecking.cpp`). This allows to logically split `Sema` in 42 groups. Table of contents added at the very beginning of `Sema`.

While grouping is intentional, as well as each group consisting of `public` declarations followed by `private` ones (without changing access in-between), exact contents and order of declarations of each group is partially carried over from old structure, partially accidental due to time constrains to do the regrouping over the weekend (`Sema` is just enormously big). Further work is expected to refine contents and order of declarations.

What is also intentional is some kind of layering, where Concepts group follows template groups, and ObjC, code completion, CUDA, HLSL, OpenACC, OpenMP, and SYCL are all placed at the end of the file, after C and C++ parts of `Sema`.

Member initializer list of `Sema` in `Sema.cpp` is rewritten to reflect new order of data members in order to avoid `-Wreorder-ctor`.