Add regression test for generic args showing host param

tests/rustdoc/const-effect-param.rs

@@ -0,0 +1,12 @@
+#![crate_name = "foo"]
+#![feature(effects, const_trait_impl)]
+
+#[const_trait]
+pub trait Tr {
+    fn f();
+}
+
+// @has foo/fn.g.html
+// @has - '//pre[@class="rust item-decl"]' 'pub const fn g<T: Tr<host>>()'
+/// foo
+pub const fn g<T: ~const Tr>() {}