Sema: fix incorrect element pointer type of c pointer array

[timholzhey]

Closes #25546, #22045 and #23506.

This code now compiles correctly:

var arr: [1]u8 = .{0};
const arr_c_ptr = @as([*c][1]u8, &arr);
arr_c_ptr.*[0] = 1;

For pointer address calculation, [*c]arr[idx] can be treated a single-item pointer *arr[idx]