Fix CodeDom so that invalid keyword identifiers are corrected

See https://bugzilla.xamarin.com/show_bug.cgi?id=17658

The current mechanism is to use an ‘_’ prefixed to the id, we could
change this to ``identifier`` if we wished, although this would require
regression testing all other uses of the (-!) function.
Merge pull request #513 from 7sharp9/master

Fix CodeDom so that invalid keyword identifiers are corrected