Auto merge of #72287 - Aaron1011:feature/min-token-collect, r=petrochenkov

Store tokens inside `ast::Expr`

This is a smaller version of #70091.

We now store captured tokens inside `ast::Expr`, which allows us to avoid some reparsing in `nt_to_tokenstream`. To try to mitigate the performance impact, we only collect tokens when we've seen an outer attribute.

This makes progress towards solving #43081. There are still many things left to do:

* Collect tokens for other AST items.
* Come up with a way to handle inner attributes (we need to be collecting tokens by the time we encounter them)
* Avoid re-parsing when a `#[cfg]` attr is used.

However, this is enough to fix spans for a simple example, which I've included as a test case.

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