Fix source range on ReturnStmt with implicit Result

If Result is implicit, but the return itself is not, we might form an
invalid SourceRange. This may not be exercised currently - I couldn't
find a way to trigger the issue from the compiler.

rdar://26638814

Author: benlangmuir

lib/AST/Stmt.cpp

@@ -153,7 +153,9 @@ SourceLoc ReturnStmt::getStartLoc() const {
   return ReturnLoc;
 }
 SourceLoc ReturnStmt::getEndLoc() const {
-  return (Result ? Result->getEndLoc() : ReturnLoc);
+  if (Result && Result->getEndLoc().isValid())
+    return Result->getEndLoc();
+  return ReturnLoc;
 }
 
 SourceLoc ThrowStmt::getEndLoc() const { return SubExpr->getEndLoc(); }