Rollup merge of #127729 - compiler-errors:ed-2024-gen, r=oli-obk

Stop using the `gen` identifier in the compiler

In preparation for edition 2024, this PR previews the fallout of removing usages of `gen` since it's being reserved as a keyword.

There are two notable changes here:
1. Had to rename `fn gen(..)` in gen/kill analysis to `gen_`. Not certain there's a better name than that.
2. There are (false?[^1]) positives in `rustc_macros` when using synstructure, which uses `gen impl` to mark an implementation. We could suppress this in a one-off way, or perhaps just ignore `gen` in macros altogether, since if an identifier ends up in expanded code then it'll get properly denied anyways.

Not relevant to the compiler, but it's gonna be really annoying to change `rand`'s `gen` fn in the library and miri...

[^1]: I haven't looked at the synstructure proc macro code itself so I'm not certain if it'll start to fail when converted to ed2024 (or, e.g., when syn starts parsing `gen` as a kw).

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