Add regression tests

(cherry picked from commit 20363f4)

tests/rustdoc/const-fn-effects.rs

@@ -0,0 +1,19 @@
+#![crate_name = "foo"]
+#![feature(effects)]
+
+// @has foo/fn.bar.html
+// @has - '//pre[@class="rust item-decl"]' 'pub const fn bar<const host: bool = true>() -> '
+/// foo
+pub const fn bar() -> usize {
+    2
+}
+
+// @has foo/struct.Foo.html
+// @has - '//*[@class="method"]' 'const fn new<const host: bool = true>()'
+pub struct Foo(usize);
+
+impl Foo {
+    pub const fn new() -> Foo {
+        Foo(0)
+    }
+}