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

This commit reverts the following commits:

- 952f90b72b3546d6b6b038d410f07ce520c59b48
- e6a0800532bb409f6d1c62f3698bdd6994a877dc (D132298)
- 176db3b3ab25ff8a9b2405f50ef5a8bd9304a6d5 (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.

NOKEYCHECK=True
GitOrigin-RevId: 342e0ebd0b7409c8808a90cf38dd45ec7dc57671
2 files changed