tests: add variance test for const traits

Added to demonstrate change in output in following commit. Many more
interesting tests change with different output, missing errors, new
errors, etc related to this but they all depend on feature flags and
are much more complex than this.

Author: davidtwco

tests/ui/traits/const-traits/variance.rs

@@ -0,0 +1,14 @@
+#![feature(rustc_attrs, const_trait_impl)]
+#![allow(internal_features)]
+#![rustc_variance_of_opaques]
+
+#[const_trait]
+trait Foo {}
+
+impl const Foo for () {}
+
+fn foo<'a: 'a>() -> impl const Foo {}
+//~^ ERROR ['a: o]
+
+fn main() {}
+

tests/ui/traits/const-traits/variance.stderr

@@ -0,0 +1,8 @@
+error: ['a: o]
+  --> $DIR/variance.rs:10:21
+   |
+LL | fn foo<'a: 'a>() -> impl const Foo {}
+   |                     ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+