Implicit conversion of concatenation result to immutable

[Per Nordlöw]

On Thursday, 1 April 2021 at 21:59:21 UTC, H. S. Teoh wrote:
> Just use .idup on the result.

That creates an unnecessary GC allocation in cases such

```d
string x;
const(char)[] y;
throw new Exception(x ~ y.idup)
```

which, imho, is very much worth considering avoiding the need for.

The implicit conversion could be special cased in the compiler to 
be allowed only on a `CatExp` being an r-value, naturally 
non-aliased.

As mentioned above this is analogous with conversion rules of new 
expressions.