Explicit default constructors are problematic, so this change removes
them from <random> and <ext/random>, as per P0935R0.

	* include/bits/random.h (linear_congruential_engine)
	(mersenne_twister_engine, subtract_with_carry_engine, random_device)
	(uniform_real_distribution, normal_distribution)
	(lognormal_distribution, gamma_distribution, chi_squared_distribution)
	(cauchy_distribution, fisher_f_distribution, student_t_distribution)
	(bernoulli_distribution, binomial_distribution,geometric_distribution)
	(negative_binomial_distribution, exponential_distribution)
	(weibull_distribution, extreme_value_distribution): Add non-explicit
	default constructors. Remove default argument for first parameter of
	explicit constructors.
	(piecewise_constant_distribution, piecewise_linear_distribution):
	Make default constructor non-explicit.
	* include/bits/uniform_int_dist.h (uniform_int_distribution): Add
	non-explicit default constructors. Remove default argument for first
	parameter of explicit constructor.
	(simd_fast_mersenne_twister_engine, beta_distribution)
	(rice_distribution, nakagami_distribution, pareto_distribution)
	(k_distribution, arcsine_distribution, hoyt_distribution)
	(triangular_distribution, von_mises_distribution)
	(hypergeometric_distribution, logistic_distribution)
	(uniform_inside_sphere_distribution): Likewise.
	(uniform_on_sphere_distribution): Make default constructor
	non-explicit.
	* testsuite/26_numerics/random/bernoulli_distribution/cons/default.cc:
	Test for non-explicit default constructor. Fix references to standard.
	* testsuite/26_numerics/random/binomial_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/cauchy_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/chi_squared_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/discrete_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/exponential_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/extreme_value_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/fisher_f_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/gamma_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/geometric_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/lognormal_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/negative_binomial_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/normal_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/piecewise_constant_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/piecewise_linear_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/poisson_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/student_t_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/uniform_int_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/uniform_real_distribution/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/weibull_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/arcsine_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/beta_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/hoyt_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/hypergeometric_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/k_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/logistic_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/nakagami_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/normal_mv_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/pareto_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/rice_distribution/cons/default.cc: Likewise.
	* testsuite/ext/random/triangular_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/uniform_inside_sphere_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/uniform_on_sphere_distribution/cons/default.cc:
	Likewise.
	* testsuite/ext/random/von_mises_distribution/cons/default.cc:
	Likewise.
	* testsuite/util/testsuite_common_types.h
	(implicitly_default_constructible): New helper.

From-SVN: r261522