In a standalone Clang build, default to building tests, like LLVM does

LLVM_INCLUDE_TESTS is set in the top-level LLVM CMakeLists, which is not being
executed when Clang is built in a separate directory.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197423 91177308-0d34-0410-b5e6-96231b3b80d8