PR middle-end/77357 - strlen of constant strings not folded

PR middle-end/77357 - strlen of constant strings not folded

gcc/ChangeLog:

	PR middle-end/77357
	PR middle-end/86428
	* builtins.c (c_strlen): Avoid out-of-bounds warnings when
	accessing implicitly initialized array elements.
	* expr.c (string_constant): Handle string initializers of
	character arrays within aggregates.
	* gimple-fold.c (fold_array_ctor_reference): Add argument.
	Store element offset.  As a special case, handle zero size.
	(fold_nonarray_ctor_reference): Same.
	(fold_ctor_reference): Add argument.  Store subobject offset.
	* gimple-fold.h (fold_ctor_reference): Add argument.

gcc/testsuite/ChangeLog:

	PR middle-end/77357
	* gcc.dg/strlenopt-49.c: New test.
	* gcc.dg/strlenopt-50.c: New test.
	* gcc.dg/strlenopt-51.c: New test.
	* gcc.dg/strlenopt-52.c: New test.

From-SVN: r262522