auto merge of #11670 : sfackler/rust/extctxt-span-note, r=alexcrichton

It was the only span_* missing.

src/libsyntax/ext/base.rs

@@ -412,6 +412,10 @@ pub fn span_bug(&self, sp: Span, msg: &str) -> ! {
         self.print_backtrace();
         self.parse_sess.span_diagnostic.span_bug(sp, msg);
     }
+    pub fn span_note(&self, sp: Span, msg: &str) {
+        self.print_backtrace();
+        self.parse_sess.span_diagnostic.span_note(sp, msg);
+    }
     pub fn bug(&self, msg: &str) -> ! {
         self.print_backtrace();
         self.parse_sess.span_diagnostic.handler().bug(msg);