Auto merge of #1405 - WizardOfArc:ISSUE_1399/add_async_to_rust_mangle, r=emilio

adds 'async' to list of matches in 'rust_mangle'

r? @emilio

I'll need to install `rustfmt` before I do anything more elaborate

Author: bors-servo

src/ir/context.rs

@@ -856,6 +856,7 @@ If you encounter an error missing from this list, please file an issue or a PR!"
                 "abstract" |
  	            "alignof" |
  	            "as" |
+ 	            "async" |
  	            "become" |
  	            "box" |
                 "break" |