PATCH rewrite duplicate_decls to not modify olddecl (fixes gcc/12336)

[Gabriel Dos Reis]

"Zack Weinberg" <zack@codesourcery.com> writes:

| I don't see any problems with your patch, however, I agree that it is
| far from obviously safe.  I'm going to test it on x86-linux and
| ia64-hpux tonight, and sparc-solaris2 after that machine finishes
| doing some other work.  I'd also appreciate a second opinion from
| another maintainer, ideally Joseph or Richard.  Provided that my
| testing goes well and that the second opinion is positive, I'm willing
| to approve this for 3.4.

duplicate_decls is, some sense, duplicated in the C++ front-end also.
What are the reasons Per's modification are not needed there?

-- Gaby, who has always been "amazed" by duplicate_decls