SIL: make sure to not crash if a function doesn't have a DebugScope

Change an assert to a bail-out condition.
I found that it _can_ happen to have a function without a debug scope.
In such a case even printing the SIL crashed.
Be a bit more tolerant.

Author: eeckstein

include/swift/SIL/SILFunction.h

@@ -1095,7 +1095,9 @@ class SILFunction
 
   /// Get the source location of the function.
   SILLocation getLocation() const {
-    assert(DebugScope && "no scope/location");
+    if (!DebugScope) {
+      return SILLocation::invalid();
+    }
     return getDebugScope()->Loc;
   }