Enable libcxx as part of the top level CMake build when it is checked
out in projects. This appears to be working on my system, and I will be
watching build bots to see if there are any issues on other platforms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191624 91177308-0d34-0410-b5e6-96231b3b80d8