go/types: export Named._Orig as Named.Origin

Export the _Orig method for Named types, and rename to Origin.  As
discussed in #47916, Orig is easily confused with Original, which is
not as good a name. Spelling out 'Origin' resolves the ambiguity.

Updates #47916

Change-Id: I377c73de82310d3f6aa0bd82dddcd60ee067155b
Reviewed-on: https://go-review.googlesource.com/c/go/+/350996
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/named.go

@@ -91,9 +91,9 @@ func (t *Named) Obj() *TypeName {
 	return t.orig.obj // for non-instances this is the same as t.obj
 }
 
-// _Orig returns the original generic type an instantiated type is derived from.
-// If t is not an instantiated type, the result is t.
-func (t *Named) _Orig() *Named { return t.orig }
+// Origin returns the parameterized type from which the named type t is
+// instantiated. If t is not an instantiated type, the result is t.
+func (t *Named) Origin() *Named { return t.orig }
 
 // TODO(gri) Come up with a better representation and API to distinguish
 //           between parameterized instantiated and non-instantiated types.