Fix "PR c++/92804 ICE trying to use concept as a nested-name-specifier"

A rather simple ICE where we failed to properly check for concept-ids
uses in nested-name-specifiers.

Tested x86_64-linux.

       /cp
       PR c++/92804
       * parser.c (cp_parser_nested_name_specifier_opt): Properly
       diagnose concept-ids.

       /testsuite
       PR c++/92804
       * g++.dg/concepts/pr92804-1.C: New.
       * g++.dg/concepts/pr92804-2.C: New.