non-global template fix POC WIP

[Suleyman]

Hello everyone.

I started working on a fix for the non-global template issue.
Progress at https://github.com/dlang/dmd/pull/9282

currently the following POC works:
```
struct S
{
     int m;

     auto add(alias a)(int b)
     {
         return a + m + b;
     }

     auto exec(alias f)()
     {
         f(m);
     }
}

void main()
{
     int a = 4;
     auto o = S(3);

     assert(o.add!a(2) == 4+3+2);

     auto dg = &o.add!a;
     assert(dg(7) == 4+3+7);

     int b;
     auto bSetter(int i) { b = i; }

     o.exec!bSetter();
     assert(b == 3);

     /* S.add instantiated to:

         auto add(int b)
         {
             return main.a + main.o.m + b;
         }

         thus only requires the frame pointer of `main`
     */
}
```

The strategy is to access the `this` variable through the frame 
pointer of the caller.