[apple] Add "umbrella" suffix to default umbrella header name

[alekseevpg]

In our project we have some both internal and external dependencies in Obj-C with manually created "umbrella" files. They are interfere with BUCK generated "ModuleName.h" umbrella files. One example of such dependency is https://github.com/yapstudios/YapDatabase/blob/master/YapDatabase/YapDatabase.h

This PR will add ModuleName"-umbrella.h" suffix to avoid this issue. This behaviour will be similar to CocoaPods

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[KapJI]

Looks like this breaks tests.