Bug 19004 – Allow passing a string literal to an extern(C) variadic parameter

A string literal usually implicitly converts to a const(char)*. This is not considered when passing one to an extern(C) variadic parameter - due to the literal being interpreted as a slice, which is disallowed. The string should be implicitly cast to a pointer, as string literals are always \0-terminated. This is handy for C string compatibility, e.g. for formatting arguments: dmd/toobj.d: f.error("must be `extern(C)` for `pragma(%s)`", isCtor ? "crt_constructor".ptr : "crt_destructor".ptr); The `.ptr` property should not be necessary.