Transform more gmp/mpfr uses to use RAII

The following picks up the coccinelle generated patch from Bernhard,
leaving out the fortran frontend parts and fixing up the rest.
In particular both gmp.h and mpfr.h contain macros like
  #define mpfr_inf_p(_x)      ((_x)->_mpfr_exp == __MPFR_EXP_INF)
for which I add operator-> overloads to the auto_* classes.

	* system.h (auto_mpz::operator->()): New.
	* realmpfr.h (auto_mpfr::operator->()): New.
	* builtins.cc (do_mpfr_lgamma_r): Use auto_mpfr.
	* real.cc (real_from_string): Likewise.
	(dconst_e_ptr): Likewise.
	(dconst_sqrt2_ptr): Likewise.
	* tree-ssa-loop-niter.cc (refine_value_range_using_guard):
	Use auto_mpz.
	(bound_difference_of_offsetted_base): Likewise.
	(number_of_iterations_ne): Likewise.
	(number_of_iterations_lt_to_ne): Likewise.
	* ubsan.cc: Include realmpfr.h.
	(ubsan_instrument_float_cast): Use auto_mpfr.