[clang][NFC] Various NFCs in CheckDefaultArgumentVisitor

Before the next patches do the following NFCs:
  - Make it a const visitor; CheckDefaultArgumentVisitor should
    really not modify the visited nodes.

  - clang-format

  - Take a reference to Sema instead of a pointer and pass it
    as the first argument to the constructor. This is for
    consistency with the other similar visitors.

  - Use range for loops when appropriate as per the style guide.

  - Use `const auto *" when appropriate as per the style guide.