Rollup merge of #73102 - petrochenkov:flatgroup, r=Aaron1011

proc_macro: Stop flattening groups with dummy spans

Reduce the scope of the hack described in https://github.com/rust-lang/rust/issues/72545#issuecomment-640276791.

We still pass AST fragments to attribute and derive macros as single nonterminal tokens rather than as tokens streams, but now use a precise flag instead of the span-based heuristic that could do lead to incorrect behavior in unrelated cases.

https://github.com/rust-lang/rust/pull/73345 attempts to fully resolve this issue, but there are some compatibility issues to be addressed.