c++: Not all character types are byte-access types.

c++: Not all character types are byte-access types.

The patch for 94923 that introduced is_byte_access_type wrongly changed
build_cplus_array_type to treat even arrays of char16_t as typeless storage,
which is wrong; only arrays of char and unsigned char have the special alias
semantics in C++.

G++ used to treat signed char the same way, as C does, but C++ has always
omitted it.

gcc/cp/ChangeLog:

	* tree.c (is_byte_access_type): Don't use char_type_p.

gcc/testsuite/ChangeLog:

	* g++.dg/Wclass-memaccess.C: Check that signed char and
	char16_t aren't treated as byte-access types.