[DDMD] Fix Issue 10724 - Allow slice of string literal to convert to const(char)*

I sorta need this for DDMD so I can strip the newline from the version file:
```d
const(char)* _version = "v" ~ import("VERSION")[0..$-1];
```

http://d.puremagic.com/issues/show_bug.cgi?id=10724