re PR c++/71537 (GCC rejects consetxpr boolean conversions and comparisons on the result of pointer arithmetic.)

	PR c++/71537
	* fold-const-call.c (fold_const_call): Handle
	CFN_BUILT_IN_{INDEX,STRCHR,RINDEX,STRRCHR}.

	* g++.dg/cpp0x/constexpr-strchr.C: New test.

From-SVN: r243284