[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

Author: riccibruno

clang/include/clang/Sema/Sema.h

@@ -11620,7 +11620,7 @@ class Sema final {
   void CheckImplicitConversions(Expr *E, SourceLocation CC = SourceLocation());
   void CheckBoolLikeConversion(Expr *E, SourceLocation CC);
   void CheckForIntOverflow(Expr *E);
-  void CheckUnsequencedOperations(Expr *E);
+  void CheckUnsequencedOperations(const Expr *E);
 
   /// Perform semantic checks on a completed expression. This will either
   /// be a full-expression or a default argument expression.