terraform: when returning a raw attribute value, use hil conversion

Because we now rely on HIL to do the computed calculation, we must make
sure the type is correct (TypeUnknown). Before, we'd just check for the
UUID in the string.

This changes all variable returns in the interpolater to run it through
`hil.InterfaceToVariable` which handles this lookup for us.

terraform/interpolate.go

@@ -404,7 +404,8 @@ func (i *Interpolater) computeResourceVariable(
 	}
 
 	if attr, ok := r.Primary.Attributes[v.Field]; ok {
-		return &ast.Variable{Type: ast.TypeString, Value: attr}, nil
+		v, err := hil.InterfaceToVariable(attr)
+		return &v, err
 	}
 
 	// computed list or map attribute

terraform/interpolate_test.go

@@ -569,6 +569,44 @@ func TestInterpolator_resourceMultiAttributesComputed(t *testing.T) {
 	})
 }
 
+func TestInterpolator_resourceAttributeComputed(t *testing.T) {
+	lock := new(sync.RWMutex)
+	// The state would never be written with an UnknownVariableValue in it, but
+	// it can/does exist that way in memory during the plan phase.
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_route53_zone.yada": &ResourceState{
+						Type: "aws_route53_zone",
+						Primary: &InstanceState{
+							ID: "z-abc123",
+							Attributes: map[string]string{
+								"id": config.UnknownVariableValue,
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+	i := &Interpolater{
+		Module:    testModule(t, "interpolate-multi-vars"),
+		StateLock: lock,
+		State:     state,
+	}
+
+	scope := &InterpolationScope{
+		Path: rootModulePath,
+	}
+
+	testInterpolate(t, i, scope, "aws_route53_zone.yada.id", ast.Variable{
+		Value: config.UnknownVariableValue,
+		Type:  ast.TypeUnknown,
+	})
+}
+
 func TestInterpolater_selfVarWithoutResource(t *testing.T) {
 	i := &Interpolater{}