go: go/types: add Object.Origin [freeze exception]

I propose to add the following methods to types.Var and types.Func:

// Origin returns the canonical object for its receiver, i.e. the object
// recorded in Info.Defs.
//
// For instantiated variables Origin returns the type-parameterized object from
// which the receiver was created. For all other variables Origin returns the
// receiver.
func (obj *Var) Origin() *Var 

// Origin returns the canonical object for its receiver, i.e. the object
// recorded in Info.Defs.
//
// For instantiated functions Origin returns the type-parameterized object from
// which the receiver was created. For all other functions, Origin returns
// receiver.
func (obj *Func) Origin() *Func

While instantiating types, we necessarily have to create instantiated copies of their methods/fields/parameters/etc. However, as we work on updating tools for generics, an emerging theme is that is useful to be able to answer the following two questions:

1. Is this object canonical?
2. What is the canonical object corresponding to this object?

Right now, these are not easy questions to answer. For methods, these can be answered by a lookup on the receiver base origin type. For fields / parameters / etc., it is more difficult and may require finding the relevant *ast.Ident and looking in Info.Defs.

Adding an Origin method to these types answers both of these questions trivially, at the cost of a pointer per object. If the added memory is a concern, there are several internal fields on types.object used during type-checking that may be externalized to free memory.

CC @griesemer @dominikh @timothy-king @mdempsky