[sycl-post-link][NFC] Various small refactorings (#4731)

- Replaced `static` functions and variables with usage of anonymous namespace.
- Renamed some functions.
- Applied clang-format to most of the `sycl-post-link.cpp` file.
- Removed some unnecessary includes, added some which were missing (like `<vector>`).
- Added `const` qualifier almost everywhere.
- Updated _some_ comments to use "entry point" instead of "kernel".
  Full replace requires renaming corresponding command line options,
  which should be done in sync with the driver - I would prefer to do so
  in a separate commit.

Author: AlexeySachkov