* Re-apply "libSyntax: Ensure round-trip printing when we build syntax tree from parser incrementally. (#12709)"
* Re-apply "libSyntax: Root parsing context should hold a reference to the current token in the parser, NFC."
* Re-apply "libSyntax: avoid copying token text when lexing token syntax nodes, NFC. (#12723)"
* Actually fix the container-overflow issue.
Since all parsing contexts need a reference to the current token of the
parser, we should pass the token reference to the root context. Therefore, the derived
sub-contexts can just copy it while being spawned.
