libstdc++: Make 16-bit std::subtract_with_carry_engine work [PR107466]

libstdc++: Make 16-bit std::subtract_with_carry_engine work [PR107466]

This implements the proposed resolution of LWG 3809, so that
std::subtract_with_carry_engine can be used with a 16-bit result_type.
Currently this produces a narrowing error when instantiating the
std::linear_congruential_engine to create the initial state. It also
truncates the default_seed constant when passing it as a result_type
argument.

Change the type of the constant to uint_least32_t and pass 0u when the
default_seed should be used.

libstdc++-v3/ChangeLog:

	PR libstdc++/107466
	* include/bits/random.h (subtract_with_carry_engine): Use 32-bit
	type for default seed. Use 0u as default argument for
	subtract_with_carry_engine(result_type) constructor and
	seed(result_type) member function.
	* include/bits/random.tcc (subtract_with_carry_engine): Use
	32-bit type for default seed and engine used for initial state.
	* testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc:
	New test.