Unset the return location for an implicit return nil

The `return` keyword here doesn't have a source
location, so unset it. This matches the other
implicit single expression returns.

Author: hamishknight

lib/Parse/ParseDecl.cpp

@@ -8732,7 +8732,7 @@ Parser::parseAbstractFunctionBodyImpl(AbstractFunctionDecl *AFD) {
         if (F->isFailable() && isa<NilLiteralExpr>(E)) {
           // If it's a nil literal, just insert return.  This is the only
           // legal thing to return.
-          auto RS = ReturnStmt::createImplicit(Context, E->getStartLoc(), E);
+          auto RS = ReturnStmt::createImplicit(Context, E);
           BS->setLastElement(RS);
           AFD->setHasSingleExpressionBody();
           AFD->setSingleExpressionBody(E);