PR c++/78551
	* constexpr.c (extract_string_elt): New.  Broken out of ...
	(cxx_eval_array_reference): ... here.  Call it.
	(cxx_eval_store_expression): Convert init by STRING_CST into
	CONSTRUCTOR, if needed.

	PR c++/78551
	* g++.dg/cpp1y/pr78551.C: New.

From-SVN: r243448