This fixes 81229, an ICE during GC.  When converting the namespace 
symbol handling I was mystified by a SET_IDENTIFIER_TYPE call that 
seemingly had no effect.  Turns out it is needed, because we've already 
set the IDENTIFIER_TYPE to the new decl's type.  Whose name points back 
at the new TYPE_DECL.  Which duplicate_decls gcc_freed when it found the 
already matched one.  So we need to change IDENTIFIER_TYPE to the old 
decl's type node.

This problem didn't manifest on a non-attributed int typedef, not sure 
why.  I suspect I'll be coming back to revisit this code as my symbol 
table overhaul continues.

nathan
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Nathan Sidwell