Add const generics feature gate test

Co-Authored-By: Gabriel Smith <yodaldevoid@users.noreply.github.com>

Author: varkor

src/test/ui/feature-gates/feature-gate-const_generics.rs

@@ -0,0 +1,6 @@
+fn foo<const X: ()>() {} //~ ERROR const generics are unstable
+//~^ const generics in any position are currently unsupported
+
+struct Foo<const X: usize>([(); X]); //~ ERROR const generics are unstable
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-const_generics.stderr

@@ -0,0 +1,25 @@
+error[E0658]: const generics are unstable (see issue #44580)
+  --> $DIR/feature-gate-const_generics.rs:1:14
+   |
+LL | fn foo<const X: ()>() {} //~ ERROR const generics are unstable
+   |              ^
+   |
+   = help: add #![feature(const_generics)] to the crate attributes to enable
+
+error[E0658]: const generics are unstable (see issue #44580)
+  --> $DIR/feature-gate-const_generics.rs:4:18
+   |
+LL | struct Foo<const X: usize>([(); X]); //~ ERROR const generics are unstable
+   |                  ^
+   |
+   = help: add #![feature(const_generics)] to the crate attributes to enable
+
+error: const generics in any position are currently unsupported
+  --> $DIR/feature-gate-const_generics.rs:1:14
+   |
+LL | fn foo<const X: ()>() {} //~ ERROR const generics are unstable
+   |              ^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.