Add a regression test for const parameters with impl Trait

src/test/ui/const-generics/apit-with-const-param.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+trait Trait {}
+
+fn f<const N: usize>(_: impl Trait) {}
+
+fn main() {}

src/test/ui/const-generics/apit-with-const-param.stderr

@@ -0,0 +1,6 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/apit-with-const-param.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+