Rollup merge of #94869 - jackh726:gats_extended, r=compiler-errors

Add the generic_associated_types_extended feature

Right now, this only ignore obligations that reference new placeholders in `poly_project_and_unify_type`. In the future, this might do other things, like allowing object-safe GATs.

**This feature is *incomplete* and quite likely unsound. This is mostly just for testing out potential future APIs using a "relaxed" set of rules until we figure out *proper* rules.**

Also drive by cleanup of adding a `ProjectAndUnifyResult` enum instead of using a `Result<Result<Option>>`.

r? `@nikomatsakis`

compiler/rustc_span/src/symbol.rs

@@ -722,6 +722,7 @@ symbols! {
         generators,
         generic_arg_infer,
         generic_associated_types,
+        generic_associated_types_extended,
         generic_const_exprs,
         generic_param_attrs,
         get_context,