Add a HIR pretty printing test for delegation.

Note that some of the output is currently bogus, with missing params and
args:
```
fn add(: _, : _) -> _ { m::add(, ) }
```
The next commit will fix this.

Author: nnethercote

tests/pretty/hir-delegation.pp

@@ -0,0 +1,23 @@
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-delegation.pp
+
+#![allow(incomplete_features)]#![feature(fn_delegation)]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+
+fn b<C>(e: C) { }
+
+trait G {
+    fn b(: _) -> _ { b({ }) }
+}
+
+mod m {
+    fn add(a: u32, b: u32) -> u32 { a + b }
+}
+
+fn add(: _, : _) -> _ { m::add(, ) }
+
+fn main() { { let _ = add(1, 2); }; }

tests/pretty/hir-delegation.rs

@@ -0,0 +1,22 @@
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-delegation.pp
+
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+
+fn b<C>(e: C) {}
+
+trait G {
+    reuse b {}
+}
+
+mod m {
+    pub fn add(a: u32, b: u32) -> u32 { a + b }
+}
+
+reuse m::add;
+
+fn main() {
+    _ = add(1, 2);
+}