[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.