c++: duplicate block-scope extern [PR 97877]

[Nathan Sidwell]

We ICED	with a duplicated block-scope extern, as duplicate_decls was
dropping the decl_lang_specific	of olddecl.  Simplys adding
appropriate retrofitting and copying turned out to be insufficient
because you can get a block-scope using	decl also matching the extern.
The latter seems a little suspicious and I have asked CWG for advice.
While there robustified the assert about releasing olddecls'
lang-specific -- if it had one,	the new	decl better have one.

         PR c++/97877
         gcc/cp/
         * decl.c (duplicate_decls): Deal with duplicated DECL_LOCAL_DECL_P
         decls.  Extend decl_lang_specific checking assert.
         gcc/testsuite/
         * g++.dg/lookup/pr97877.C: New.


pushing to trunk

-- 
Nathan Sidwell
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