dip1000 and preview in combine to cause extra safety errors

[Steven Schveighoffer]

```d
string foo(in string s)
{
     return s;
}

void main()
{
     import std.stdio;
     string[] result;
     foreach(c; "hello")
     {
         result ~= foo([c]);
     }
     writeln(result);
}
```

With no previews, preview=dip1000, or preview=in, this outputs: `["h", 
"e", "l", "l", "o"]`

With both preview=dip1000 and preview=in, this outputs: `["o", "o", "o", 
"o", "o"]`

What is happening is the compiler is somehow convinced that it can 
allocate the array literal on the stack (and overwrites that literal 
each loop).

I know this isn't `@safe` code, but `@system` code shouldn't be made 
less safe by the preview switches!

I know people write `in` instead of `const` all the time *simply because 
it's shorter*.

Thoughts?

-Steve