[CMake] Move CLANG_INCLUDE_TESTS in advance of add_subdirectory(tools).

The target "check-clang-tools" is affected by CLANG_INCLUDE_TESTS but it was undefined in 1st configuration procedure.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249283 91177308-0d34-0410-b5e6-96231b3b80d8