…lity, r=petrochenkov

Recursively expand `TokenKind::Interpolated` in `probably_equal_for_proc_macro`

Fixes rust-lang#68430

When comparing the captured and re-parsed `TokenStream` for a `TokenKind::Interpolated`, we currently treat any nested `TokenKind::Interpolated` tokens as unequal. If a `TokenKind::Interpolated` token shows up in the captured `TokenStream` due to a `macro_rules!` expansion, we will throw away the captured `TokenStream`, losing span information.

This PR recursively invokes `nt_to_tokenstream` on nested `TokenKind::Interpolated` tokens, effectively flattening the stream into a sequence of non-interpolated tokens. This allows it to compare equal with the re-parsed stream, allowing us to keep the original captured `TokenStream` (with span information).

This requires all of the `probably_equal_for_proc_macro` methods to be moved from `librustc_ast` to `librustc_parse` so that they can call `nt_to_tokenstream`.

…hievink

small select cleanup