Advanced const propagation for structs

[Ilya Yaroshenko via Digitalmars-d]

Hi,

Extern precompiled Mir GLAS requires additional API changes.
Reduced example:

```d
struct S(T)
{
   size_t len;
   T ptr;
}

auto foo(S!(const(double)*) sl)
{
}

S!(double*) a;
const S!(double*) b;

foo(a); // fails
foo(b); // fails

```

https://issues.dlang.org/show_bug.cgi?id=16616