Unity: Enable UNITY_BUILD for OBJC and OBJCXX files

Rob Raguet-Schofieldrequested to merge
ragfield/cmake:UNITY_BUILD_for_OBJC_and_OBJCXX into master

UNITY_BUILD currently only works with C and CXX files. This patch allows it to also work with OBJC and OBJCXX files.

Topic-rename: unity-build-objc

Edited by Brad King

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