c++: Poor diagnostic in header-unit [PR 99468]

[Nathan Sidwell]

We didn't specifically check for a module-decl inside a	header unit.
  That leads to a confusing diagostic.  Fixed thusly.

         gcc/cp/
         * lex.c (module_token_filter::resume): Ignore module-decls inside
         header-unit.
         * parser.c (cp_parser_module_declaration): Reject in header-unit.
         gcc/testsuite/
         * g++.dg/modules/pr99468.H: New.
-- 
Nathan Sidwell
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