c++: Add test for DR 1423, Convertibility of nullptr to bool.

c++: Add test for DR 1423, Convertibility of nullptr to bool.

DR 1423, which supersedes DR 654, says that you can't copy-init
a bool from a std::nullptr_t:

  bool b = nullptr;  // error

Conversely, it works with direct-initialization which is more
permissive than copy-initialization.

No code changes necessary since we handle it right.

2020-02-26  Marek Polacek  <polacek@redhat.com>

	DR 1423, Convertibility of nullptr to bool.
	* g++.dg/DRs/dr1423.C: New test.