Allow using dig.As with dig.Group (#375)

This PR adds the ability to use dig.As with dig.Group.
When dig.As is used with dig.Group, the value produced
by the constructor will be provided to the specified group
as the type specified by dig.As.

```
 c.Provide(newBuffer, dig.As(new(io.Reader)), dig.Group("readers"))
```

For example, the above code is equivalent to the following.

```
 c.Provide(func(...) io.Reader {
   b := newBuffer(...)
   return b
 }, dig.Group("readers"))
```

Fixes #341

decorate_test.go

@@ -32,6 +32,27 @@ import (
 	"go.uber.org/dig/internal/digtest"
 )
 
+type myInt interface {
+	String() string
+	Increment()
+}
+type someInt int
+
+var _ myInt = (*someInt)(nil)
+
+func newSomeInt(i int) *someInt {
+	v := someInt(i)
+	return &v
+}
+
+func (i *someInt) String() string {
+	return fmt.Sprintf("%d", i)
+}
+
+func (i *someInt) Increment() {
+	*i += 1
+}
+
 func TestDecorateSuccess(t *testing.T) {
 	t.Run("simple decorate without names or groups", func(t *testing.T) {
 		t.Parallel()
@@ -124,6 +145,36 @@ func TestDecorateSuccess(t *testing.T) {
 		})
 	})
 
+	t.Run("decorate grouped values provided as", func(t *testing.T) {
+		t.Parallel()
+
+		type A struct {
+			dig.In
+			Values []myInt `group:"values"`
+		}
+
+		type B struct {
+			dig.Out
+			Values []myInt `group:"values"`
+		}
+
+		c := digtest.New(t)
+
+		for i := range make([]int, 3) {
+			i := i
+			c.RequireProvide(func() *someInt { return newSomeInt(i) }, dig.Group("values"), dig.As(new(myInt)))
+		}
+		c.Decorate(func(in A) B {
+			for _, v := range in.Values {
+				v.Increment()
+			}
+			return B{Values: in.Values}
+		})
+		c.RequireInvoke(func(a A) {
+			assert.ElementsMatch(t, []myInt{newSomeInt(1), newSomeInt(2), newSomeInt(3)}, a.Values)
+		})
+	})
+
 	t.Run("value groups with multiple decorations", func(t *testing.T) {
 		type params struct {
 			dig.In