swift234

fix <rdar://problem/17686667> If super.init is implicitly inserted, DI diagnostics have no location info

This has two changes:
 - In the case when we have an invalid source loc, DI will default to emitting diagnostics
   at the enclosing function's location.  This makes it more robust in general against
   implicitly synthesized code.

 - For this diagnostic in particular, emit a tailored message, since super.init is a commonly
   synthesized thing.



Swift SVN r20104