Auto merge of #49252 - Manishearth:easy-feature-flag, r=nikomatsakis

Easy edition feature flag

We no longer gate features on epochs; instead we have a `#![feature(rust_2018_preview)]` that flips on a bunch of features (currently dyn_trait).

Based on #49001 to avoid merge conflicts

r? @nikomatsakis

src/test/run-pass/epoch-gate-feature.rs

@@ -11,7 +11,7 @@
 // Checks if the correct registers are being used to pass arguments
 // when the sysv64 ABI is specified.
 
-// compile-flags: -Zedition=2018
+#![feature(rust_2018_preview)]
 
 pub trait Foo {}