Fix two discrepancies between the cited snippets and the full code.

There were quite a few places in the tutorial where the snippets were not up to date with the full code listings given. I have updated all of the ones I could find, which mostly involves changing `.` to `->` when accessing through a `std::unique_ptr`, changing `while (1)` to `while (true)`, and other such small changes.

There are still however a few places where I am not sure what to do, such as:
- Chapter 4: `ParseTopLevelExpr()` in chapter 3 sets the ProtoTypeAST
             name to "", however it is referred to as "__anon_expr"
             in chapter 4. Would it be required to mention this change
             in chapter 4?
- Chapter 9: The code snippets refer to the top level expression as
             "main", however the full code listing refers to it as
             "__anon_expr". I think given the context of the chapter
             it makes sense to refer to it as "main", so I have
             updated the code listing to reflect that.
- Chapter 9: In chapter 9 the diff given for `HandleTopLevelExpression()`
             deletes code that is not mentioned anywhere else, so I am
             not sure what is to be done there.
- Miscellaneous: I don't think this is very important, however the
                 casing used for the first word of error messages
                 tends to vary between upper and lower case between
                 chapters and I do not know if it is worth reconciling
                 these differences.

Reviewed By: xgupta

Differential Revision: https://reviews.llvm.org/D142323

* replace virtual with override
* use default like in full code example

Differential Revision: https://reviews.llvm.org/D123110

Use "operands" instead of "operators"

Closes #54397

Several links in this document referred to `LangImpl4.html` or
`LangImpl7.html`. However, now these pages use two digits, so for these
links to function they need to be modified to `LangImpl04.html`, and so
on -- note the extra `0`.