Define CMake option CLANG_INCLUDE_TESTS *before* traversing into tests/ subdirectory. Otherwise, while configuring the build tree for the first time, Clang unit tests could avoid being added to 'check-clang' command, and thus avoid being built and executed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176080 91177308-0d34-0410-b5e6-96231b3b80d8