Enable method recognition by generic type constraint

[ijsgaus]

This is not possible now:

pubic class A  { }
public class B : A { }
public class C : A { }
public static class Ex
{
      public static TB Do<TB>(this TB tb) where TB : B  => return tb;
      public static TC Do<TC>(this TC tc) where TC : C => return tc;
}

But can be resolved in compile time as special naming convention.

[bondsbw]

Related:

#98 and dotnet/roslyn#250
dotnet/roslyn#7459
dotnet/roslyn#6075