Correct handling of constant representations containing embedded nuls (backport from...

Correct handling of constant representations containing embedded nuls (backport from trunk).

Resolves:
PR middle-end/95189 - memcmp being wrongly stripped like strcm
PR middle-end/95886 - suboptimal memcpy with embedded zero bytes

gcc/ChangeLog:

	PR middle-end/95189
	PR middle-end/95886
	* builtins.c (inline_expand_builtin_string_cmp): Rename...
	(inline_expand_builtin_bytecmp): ...to this.
	(builtin_memcpy_read_str): Don't expect data to be nul-terminated.
	(expand_builtin_memory_copy_args): Handle object representations
	with embedded nul bytes.
	(expand_builtin_memcmp): Same.
	(expand_builtin_strcmp): Adjust call to naming change.
	(expand_builtin_strncmp): Same.
	* expr.c (string_constant): Create empty strings with nonzero size.
	* fold-const.c (c_getstr): Rename locals and update comments.
	* tree.c (build_string): Accept null pointer argument.
	(build_string_literal): Same.
	* tree.h (build_string): Provide a default.
	(build_string_literal): Same.

gcc/testsuite/ChangeLog:

	PR middle-end/95189
	PR middle-end/95886
	* gcc.dg/memcmp-pr95189.c: New test.
	* gcc.dg/strncmp-3.c: New test.
	* gcc.target/i386/memcpy-pr95886.c: New test.