c++: global-namespace-qualified var after class def [PR90107]

c++: global-namespace-qualified var after class def [PR90107]

Here we wrongly reject the definition of "::N::a"

  struct A;
  namespace N { extern A a; }
  struct A {} ::N::a;

because our code to diagnose a missing ; after a class definition doesn't
realize that :: can follow a class definition.

	PR c++/90107

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_class_specifier_1): Accept :: after a class
	definition.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/qualified6.C: New test.