Add regression test for link generation on foreign macro in jump to defintion feature

tests/rustdoc/auxiliary/jump-to-def-macro.rs

@@ -0,0 +1,6 @@
+#[macro_export]
+macro_rules! symbols {
+    ($name:ident = $value:expr) => {
+        pub const $name: isize = $value;
+    }
+}

tests/rustdoc/jump-to-def-macro.rs

@@ -0,0 +1,15 @@
+//@ aux-build:jump-to-def-macro.rs
+//@ build-aux-docs
+//@ compile-flags: -Zunstable-options --generate-link-to-definition
+
+#![crate_name = "foo"]
+
+// @has 'src/foo/jump-to-def-macro.rs.html'
+
+#[macro_use]
+extern crate jump_to_def_macro;
+
+// @has - '//a[@href="../../jump_to_def_macro/macro.symbols.html"]' 'symbols!'
+symbols! {
+    A = 12
+}