Avoid returning original macro if expansion fails.

Closes #11692. Instead of returning the original expression, a dummy expression
(with identical span) is returned. This prevents infinite loops of failed
expansions as well as odd double error messages in certain situations.

src/libsyntax/ext/asm.rs

@@ -64,7 +64,7 @@ pub fn expand_asm(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                                                    "inline assembly must be a string literal.") {
                     Some((s, st)) => (s, st),
                     // let compilation continue
-                    None => return MacResult::dummy_expr(),
+                    None => return MacResult::dummy_expr(sp),
                 };
                 asm = s;
                 asm_str_style = Some(style);