Revert the removal of LLVM_ENABLE_PROJECTS for libc++ and libc++abi

This commit reverts the following commits:

- 952f90b7
- e6a08005 (D132298)
- 176db3b3 (D132324)

These commits caused CI instability and need to be reverted in order
to figure things out again. See the discussion in https://llvm.org/D132324
for more information.