Ensure Parent ConfigurationClass loaded on overrides

@cbeams @jhoeller This is a fix for [SPR-10546](https://jira.springsource.org/browse/SPR-10546), but I would like a few extra set of eyes on it before pushing. Do you mind reviewing this? Ideally I'd like to get this in 4.0.0.M2 and 3.2.3 as this situation will be quite prevalent when using the Spring Security Java Config.

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Previously ConfigurationClassParser could override a nested
@Configuration without consideration of @Bean's defined in parent
classes.

This commit ensures that if the original ConfigurationClass contains
additional bean definitions it is processed again.

Issue: SPR-10546