re PR c++/84724 (internal compiler error: in single_succ_edge, at basic-block.h:339 with a declaration of __builtin_trap)

	PR c++/84724
	* decl.c (duplicate_decls): Don't override __* prefixed builtins
	except for __[^b]*_chk, instead issue permerror and for -fpermissive
	also a note and return olddecl.

	* g++.dg/ext/pr84724.C: New test.

From-SVN: r258391