Auto merge of #1031 - emilio:warn-stderr, r=fitzgen

lib: Print warnings to stderr instead of stdout.

Seems like the right thing to do, and allows me to test stuff piping to rustc instead of saving to a file.

Author: bors-servo

src/lib.rs

@@ -1692,7 +1692,7 @@ fn parse(context: &mut BindgenContext) -> Result<(), ()> {
     for d in context.translation_unit().diags().iter() {
         let msg = d.format();
         let is_err = d.severity() >= CXDiagnostic_Error;
-        println!("{}, err: {}", msg, is_err);
+        eprintln!("{}, err: {}", msg, is_err);
         any_error |= is_err;
     }