Some more improvements to BasicFormat + misc changes

[ahoppen]

This is basically a grab bag of changes that I want to do before my final PR to improve BasicFormat. Splitting it so it’s easier to review:

Contains a couple of bug fixes:

• Fix an access to freed memory when modifying token nodes
  • When a token node is modified, we would not add its arena as a child to the new arena. This is unsafe because the newly created token might still have its text stored in the original arena.
• Fix a bug where allTokens would not include the last token if it is missing

Minor new features

• Add a walk method to SyntaxRewriter
  • Does the same as visit but also re-attaches the rewritten node into the parent tree (I will need this in the next PR)

Biggest change:

• Add placeholder children to all missing syntax nodes
  • It’s much easier to get correct formatting in the new BasicFormat if the MissingExprSyntax etc. node has a token inside of it. Because if it does, we know whether we should e.g. add a space after try in front of a MissingExprNode. This also required a few small changes to SwiftParserDiagnostics

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows

[ahoppen]

@bnbarham @rintaro I’m merging this now because I’ve got two more PRs that depend on this and which I would like to get merged ASAP as well. I‘ll address review comments in a follow-up PR.