Auto merge of #97251 - petrochenkov:eqtokens, r=nnethercote

rustc_parse: Move AST -> TokenStream conversion logic to rustc_ast

In the past falling back to reparsing pretty-printed strings was common, so some of this logic had to live in `rustc_parse`, but now the reparsing fallback is only used in two corner cases so we can move this logic to `rustc_ast` which makes many things simpler.

It also helps to fix `MacArgs::inner_tokens` for `MacArgs::Eq` with non-literal expressions, which is done in the second commit.
r? `@nnethercote`

Author: bors