Add test for issue 6607

Author: CPunisher

tests/source/issue-6607/fn-type.rs

@@ -0,0 +1,3 @@
+struct Foo {
+    v: fn(#[cfg(false)] i32),
+}

tests/target/issue-6607/fn-type.rs

@@ -0,0 +1,3 @@
+struct Foo {
+    v: fn(#[cfg(false)] i32),
+}