[Darwin] Exclude interception union tests on Darwin and Android

Since the directory is empty on Darwin, disable the inclusion and avoid
the warning below. Exclude on Android as well to match the behavior from
lib/interception/tests/CMakeLists.txt

lit.py:
/Users/buildslave/jenkins/sharedspace/clang-R_master@2/llvm/utils/lit/lit/discovery.py:224:
warning: input
'/Users/buildslave/jenkins/sharedspace/clang-R_master@2/clang-build/Build/tools/clang/runtime/compiler-rt-bins/test/interception/Unit'
contained no tests

This fixes the above warning in some of public bots, like
http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto_check/8686

Differential Revision: https://reviews.llvm.org/D23128

rdar://problem/27581108

llvm-svn: 277692