c++: Diagnose a deduction guide in a wrong scope [PR91759]

The following testcase is accepts-invalid since r7-6608-ga56c0ac08242269b.
Before that change we had this
"deduction guide %qD must be declared in the same scope as %qT"
diagnostics for it, after the change it is expected to be diagnosed
in set_decl_namespace at the not_found: label in there.  On this testcase
nothing is diagnosed though, because set_decl_namespace isn't called at all,
as in_namespace is NULL.

The following patch restores the old warning but does it only in case we
don't call set_decl_namespace.

2020-03-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/91759
	* decl.c (grokfndecl): Restore old diagnostics about deduction
	guide declared in different scope if in_namespace is NULL_TREE.

	* g++.dg/cpp1z/class-deduction72.C: New test.