constexpr.c (cxx_fold_indirect_ref): Use similar_type_p.

	* constexpr.c (cxx_fold_indirect_ref): Use similar_type_p.

Merging the similar_type_p change to the concepts branch broke a cmcstl2
testcase; investigating led me to this small testcase which has always
failed on trunk.

	(cxx_eval_indirect_ref): Likewise.  Improve error location.

From-SVN: r276192