[RFC] Remove support for building C++ with `LLVM_ENABLE_PROJECTS`

This has been officially deprecated since D112724, meaning the
deprecation warning is present in released 14 and 15.

This makes me think that now, shortly after the 15 release is branched,
is a good time to pull the trigger.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D132324