[codegen/hcl2] Fix assignability from dynamic.

Types like output(T), promise(T), and union(T_0, ..., T_N) should be
assignable from dynamic if they contain an element type that is
assignable from dynamic.

This allows consumers to use the following code to check if some type
behaves like the dynamic type w.r.t. conversions:

```
if t.AssignableFrom(model.DynamicType) {
}
```

Fixes #4703.

pkg/codegen/hcl2/model/type.go

@@ -61,13 +61,7 @@ var (
 )
 
 func assignableFrom(dest, src Type, assignableFrom func() bool) bool {
-	if dest.Equals(src) || dest == DynamicType {
-		return true
-	}
-	if src == DynamicType {
-		return false
-	}
-	return assignableFrom()
+	return dest.Equals(src) || dest == DynamicType || assignableFrom()
 }
 
 func conversionFrom(dest, src Type, unifying bool, conversionFrom func() ConversionKind) ConversionKind {

pkg/codegen/hcl2/model/type_test.go

@@ -53,6 +53,28 @@ func TestDynamicType(t *testing.T) {
 		"int":  IntType,
 	})))
 
+	// Test that DynamicType is assignable to certain types and not assignable to others.
+	assert.True(t, NewOptionalType(DynamicType).AssignableFrom(DynamicType))
+	assert.True(t, NewOutputType(DynamicType).AssignableFrom(DynamicType))
+	assert.True(t, NewPromiseType(DynamicType).AssignableFrom(DynamicType))
+	assert.True(t, NewUnionType(BoolType, DynamicType).AssignableFrom(DynamicType))
+
+	assert.False(t, BoolType.AssignableFrom(DynamicType))
+	assert.False(t, IntType.AssignableFrom(DynamicType))
+	assert.False(t, NumberType.AssignableFrom(DynamicType))
+	assert.False(t, StringType.AssignableFrom(DynamicType))
+
+	assert.False(t, NewOptionalType(BoolType).AssignableFrom(DynamicType))
+	assert.False(t, NewOutputType(BoolType).AssignableFrom(DynamicType))
+	assert.False(t, NewPromiseType(BoolType).AssignableFrom(DynamicType))
+	assert.False(t, NewMapType(BoolType).AssignableFrom(DynamicType))
+	assert.False(t, NewListType(BoolType).AssignableFrom(DynamicType))
+	assert.False(t, NewUnionType(BoolType, IntType).AssignableFrom(DynamicType))
+	assert.False(t, NewObjectType(map[string]Type{
+		"bool": BoolType,
+		"int":  IntType,
+	}).AssignableFrom(DynamicType))
+
 	// Test that DynamicType is convertible from any type.
 	assert.True(t, DynamicType.ConversionFrom(BoolType).Exists())
 	assert.True(t, DynamicType.ConversionFrom(IntType).Exists())