Add test for following ResolvedTypeRef

Author: Nashenas88

libbindgen/tests/expectations/tests/resolved_type_def_function.rs

@@ -0,0 +1,7 @@
+/* automatically generated by rust-bindgen */
+
+
+#![allow(non_snake_case)]
+
+
+pub type FuncType = ::std::option::Option<unsafe extern "C" fn()>;

libbindgen/tests/headers/resolved_type_def_function.h

@@ -0,0 +1,2 @@
+typedef void (FuncType) ();
+extern FuncType Func;