[CMake] Fixing errors in r361513

Summary:
I somehow messed this up. libcxx appends the subdirectories itself, so we don't need to add them here.

Also, r361513 broke the "projects" build of libcxx because it always included the extra targets.

Reviewers: lebedev.ri, mclow.lists

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62336

llvm-svn: 361535