go/types: update comment to refer to package go/constant

For #11949.

Change-Id: I4329604a24efc7f40cf5bf52fb3c9e30916b3cc2
Reviewed-on: https://go-review.googlesource.com/12931
Reviewed-by: Alan Donovan <[email protected]>

Author: griesemer

src/go/types/api.go

@@ -14,8 +14,8 @@
 // language object (Object) it denotes.
 // Use Info.{Defs,Uses,Implicits} for the results of name resolution.
 //
-// Constant folding computes the exact constant value (exact.Value) for
-// every expression (ast.Expr) that is a compile-time constant.
+// Constant folding computes the exact constant value (constant.Value)
+// for every expression (ast.Expr) that is a compile-time constant.
 // Use Info.Types[expr].Value for the results of constant folding.
 //
 // Type inference computes the type (Type) of every expression (ast.Expr)
@@ -218,7 +218,7 @@ func (info *Info) ObjectOf(id *ast.Ident) Object {
 type TypeAndValue struct {
 	mode  operandMode
 	Type  Type
-	Value exact.Value
+	Value exact.Value // == constant.Value
 }
 
 // TODO(gri) Consider eliminating the IsVoid predicate. Instead, report