[ref variables] Allow `auto ref`

[IchorDev]

On Monday, 29 July 2024 at 15:26:39 UTC, Quirin Schroll wrote:
> In particular in generic code, `auto ref` variables that infer 
> `ref` if and only if the initializer is an lvalue could be 
> quite valuable. The same is true for `foreach` over ranges, 
> where the foreach variable would infer `ref` if `front` is an 
> lvalue (be it a variable or a function that returns by 
> reference) and lastly, for conditions, `if (auto ref x = f())` 
> could infer `ref` from the value category of the initializer, 
> like any other variable.

This makes a lot of sense. I’d also love it if `auto ref` worked 
inside all templates, not just template functions. Often I want 
to do this:
```d
struct S(T){
   this(auto ref T x){}
}
```
But I’d get an error:
```d
S!long x; //Error: cannot explicitly instantiate template 
function with `auto ref` parameter
```
If I make `this` a template function it works. For template 
functions, even explicit instantiation works just fine. This 
requirement feels rather silly and leads to a lot of unnecessary 
parameter-less templates.