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.
auto merge of #12370 : rcxdude/rust/macro_fix, r=alexcrichton

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.

This is a slightly better fix than #12197, because it does not produce a double error and also fixes a few other cases where an infinite loop could happen.

This does not fix the other issue in #11692 (non-builtin macros not being recognised when expanded inside macros), which I think should be moved into a separate issue.