PR tree-optimization/87756 - missing unterminated argument warning using address of a constant character

gcc/ChangeLog:

	PR tree-optimization/87756
	* expr.c (string_constant): Synthesize a string literal from
	the address of a constant character.
	* tree.c (build_string_literal): Add an argument.
	* tree.h (build_string_literal): Same.

gcc/testsuite/ChangeLog:

	PR tree-optimization/87756
	* gcc.dg/builtin-memchr-2.c: New test.
	* gcc.dg/builtin-memchr-3.c: Same.
	* gcc.dg/warn-sprintf-no-nul-2.c: Same.

From-SVN: r266418