eval.c:evaluate_subexp_standard: Use RAII to avoid leaks
    
    While working on the no-debug-info debugging improvements series, I
    noticed these bare xfree calls, which lead to leaks if
    evaluate_subexp_standard throws.
    
    Fix that by reworking make_params as a RAII class.  Ends up
    eliminating a couple heap allocations too.
    
    gdb/ChangeLog:
    2017-09-20  Pedro Alves  <palves@redhat.com>
    
    	* eval.c (make_params): Delete, refactored as ...
    	(class fake_method): ... this new type's ctor.
    	(fake_method::~fake_method): New.
    	(evaluate_subexp_standard): Use 'fake_method'.