Traverse interpolated TTs so we can parse them as other things

[kmcallister]

See discussion on #9364.

[pczarn]

the problem is that [interpolated statements] cannot be broken apart and re-parsed after they have been parsed once.

The ability to convert token-trees ↔ AST is also needed to fix #16472.

[kmcallister]

Do we need that ability here? It's only about interpolated tt matches.

[jseyfried]

Fixed in #34908.