Auto merge of #85707 - jam1garner:future_prelude_collision_lint, r=nikomatsakis

Add `future_prelude_collision` lint

Implements #84594. (RFC rust-lang/rfcs#3114 ([rendered](https://github.com/rust-lang/rfcs/blob/master/text/3114-prelude-2021.md))) Not entirely complete but wanted to have my progress decently available while I finish off the last little bits.

Things left to implement:

* [x] UI tests for lints
* [x] Only emit lint for 2015 and 2018 editions
* [ ] Lint name/message bikeshedding
* [x] Implement for `FromIterator` (from best I can tell, the current approach as mentioned from [this comment](https://github.com/rust-lang/rust/issues/84594#issuecomment-847288288) won't work due to `FromIterator` instances not using dot-call syntax, but if I'm correct about this then that would also need to be fixed for `TryFrom`/`TryInto`)*
* [x] Add to `rust-2021-migration` group? (See #85512) (added to `rust-2021-compatibility` group)
* [ ] Link to edition guide in lint docs

*edit: looked into it, `lookup_method` will also not be hit for `TryFrom`/`TryInto` for non-dotcall syntax. If anyone who is more familiar with typecheck knows the equivalent for looking up associated functions, feel free to chime in.

compiler/rustc_span/src/symbol.rs

@@ -586,6 +586,7 @@ symbols! {
         from,
         from_desugaring,
         from_generator,
+        from_iter,
         from_method,
         from_output,
         from_residual,
@@ -1238,7 +1239,9 @@ symbols! {
         truncf32,
         truncf64,
         try_blocks,
+        try_from,
         try_from_trait,
+        try_into,
         try_into_trait,
         try_trait_v2,
         tt,