eval.c:evaluate_subexp_standard: Use RAII to avoid leaks

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'.