[Sema] SequenceChecker: Add some comments + related small NFCs

NFCs factored out of the following patches:
- Change all of the `Expr *` to `const Expr *` in SequenceChecker for
  const-correctness. SequenceChecker should not modify AST nodes.
- Add some comments.
- clang-format

Differential Revision: https://reviews.llvm.org/D57659

Reviewed By: xbolva00