Added some methods to improve the handling of auto_mapping feature
feature #11815 Added some methods to improve the handling of auto_mapping feature (goetas)

This PR was squashed before being merged into the 2.6-dev branch (closes #11815).

Discussion
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Added some methods to improve the handling of auto_mapping feature

| Q             | A
| ------------- | ---
| Bug fix?      |no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?  | yes
| Fixed tickets | https://github.com/doctrine/DoctrineBundle/issues/60
| License       | MIT

This PR is a part that is required by:
* https://github.com/doctrine/DoctrineMongoDBBundle/pull/267
* https://github.com/doctrine/DoctrineBundle/pull/321

The proposed PRs are an alternative to https://github.com/symfony/symfony/pull/11650 and https://github.com/doctrine/DoctrineBundle/pull/322

Commits
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2e30a43 Added some methods to improve the handling of auto_mapping feature