go/types: expose more API under -tags=typeparams

Updates #44933.

Change-Id: I0c4c2a54f67b47771f4fa59f11c47fa7b0dde799
Reviewed-on: https://go-review.googlesource.com/c/go/+/317029
Trust: Matthew Dempsky <[email protected]>
Trust: Robert Griesemer <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: mdempsky

src/go/types/api_typeparams.go

@@ -11,7 +11,11 @@ import (
 	"go/ast"
 )
 
-type Inferred = _Inferred
+type (
+	Inferred  = _Inferred
+	Sum       = _Sum
+	TypeParam = _TypeParam
+)
 
 func GetInferred(info *Info) map[ast.Expr]Inferred {
 	return info._Inferred
@@ -20,3 +24,16 @@ func GetInferred(info *Info) map[ast.Expr]Inferred {
 func SetInferred(info *Info, inferred map[ast.Expr]Inferred) {
 	info._Inferred = inferred
 }
+
+func NewSum(types []Type) Type { return _NewSum(types) }
+
+func (s *Signature) TParams() []*TypeName           { return s._TParams() }
+func (s *Signature) SetTParams(tparams []*TypeName) { s._SetTParams(tparams) }
+
+func (t *Interface) HasTypeList() bool  { return t._HasTypeList() }
+func (t *Interface) IsComparable() bool { return t._IsComparable() }
+func (t *Interface) IsConstraint() bool { return t._IsConstraint() }
+
+func (t *Named) TParams() []*TypeName { return t._TParams() }
+func (t *Named) TArgs() []Type        { return t._TArgs() }
+func (t *Named) SetTArgs(args []Type) { t._SetTArgs(args) }