Fix quick fix for isolatedDeclarations to keep trailing unknown in generics

[blickly]

Update typeToMinimizedReferenceType helper to avoid dropping trailing unknown inside generics. This avoids a bug where non-optional trailing unknown types were incorrectly dropped.

Fixes #61208

[jakebailey]

Would a better fix to just be to check to see how many required type args are needed, and then not cross that threshold? I would think we have a way to access that information.

[jakebailey]

Yeah, this looks better!

[blickly]

Great point! I misremembered what the constraints were when I assumed that we couldn't look up which type arguments were required or not. AFAICT, having a constraint corresponds to being optional, which seems like a more direct way to check than I was doing before.