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.

gcc/testsuite/ChangeLog

 2020-02-26  Marek Polacek  <polacek@redhat.com>
 
+	DR 1423, Convertibility of nullptr to bool.
+	* g++.dg/DRs/dr1423.C: New test.
+
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
 	* g++.dg/cpp2a/constexpr-init16.C: New test.
 	* g++.dg/cpp2a/constexpr-init17.C: New test.

gcc/testsuite/g++.dg/DRs/dr1423.C

+// DR 1423 - Convertibility of nullptr to bool.
+// { dg-do compile { target c++11 } }
+
+bool b = nullptr; // { dg-error "converting to .bool. from .std::nullptr_t. requires direct-initialization" }
+bool b2(nullptr);
+bool b3{nullptr};
+bool b4 = { nullptr }; // { dg-error "converting to .bool. from .std::nullptr_t. requires direct-initialization" }