Inconsistent handling of directives in buildASTSchema and buildClientSchema

I noticed a difference in how specified (standard) directives are added by `buildASTSchema` vs `buildClientSchema`.

In the former function, the specified directives [are added](https://github.com/graphql/graphql-js/blob/90bd6ff72625173dd39a1f82cfad9336cfad8f65/src/utilities/buildASTSchema.ts#L83) automatically if they are missing.

In the latter function, the specified directives [are not added](https://github.com/graphql/graphql-js/blob/90bd6ff72625173dd39a1f82cfad9336cfad8f65/src/utilities/buildClientSchema.ts#L105) automatically if they are missing.

I think this should be handled in the same way by both method - this different behavior caused some head-scratching [here](https://github.com/graphql-python/gql/issues/278).

The main problem is `buildClientSchema` *not* adding the specified directives. Maybe we can change this to add them, first optionally, and then in the next major version by default?