The reason this is not enabled by default is because there is no way for
Clang to guarantee that the LLVM unit testing infrastruture has been built.
However, if it /has/ been built, there's no reason why the standalone Clang
build can't use it!

This should have no effect on existing builds -- in a combined build the
value of the CLANG_INCLUDE_TESTS option defaults to the LLVM equivalent,
and in a standalone build it defaults to off.

llvm-svn: 174691