c++: Fix bogus warning in deprecated namespace [PR99318]

c++: Fix bogus warning in deprecated namespace [PR99318]

In GCC 10, I introduced cp_warn_deprecated_use_scopes so that we can
handle attribute deprecated on a namespace declaration.  This
function walks the decl's contexts so that we warn for code like

  namespace [[deprecated]] N { struct S { }; }
  N::S s;

We call cp_warn_deprecated_use_scopes when we encounter a TYPE_DECL.
But in the following testcase we have a TYPE_DECL whose context is
a deprecated function; that itself is not a reason to warn.  This
patch limits for which entities we call cp_warn_deprecated_use;
essentially it's what can follow ::.

I noticed that we didn't test that

  struct [[deprecated]] S { static void fn(); };
  S::fn();

produces the expected warning, so I've added gen-attrs-73.C.

gcc/cp/ChangeLog:

	PR c++/99318
	* decl2.c (cp_warn_deprecated_use_scopes): Only call
	cp_warn_deprecated_use when decl is a namespace, class, or enum.

gcc/testsuite/ChangeLog:

	PR c++/99318
	* g++.dg/cpp0x/attributes-namespace6.C: New test.
	* g++.dg/cpp0x/gen-attrs-73.C: New test.