[ref variables] Allow `auto ref`

[Quirin Schroll]

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.

In my opinion, this is an obvious no-brainer because generic code 
can’t know if a type is expensive to copy or even copyable, and 
taking lvalues by reference and rvalues by value is the obvious 
right way.

In contrast to function parameters, `auto ref` for local 
variables, foreach variables, and if condition variables requires 
no template context and could be a way for a programmer to 
express: Just store the value, don’t (needlessly) copy it, 
similar to C++’s `auto&&`, which, too, works for local variables, 
loop variables, etc. and not only function template parameters.

The same way `__traits(isRef)` works for `auto ref` function 
template parameters, it should work for local variables, too.