[CMake] Change -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=off to -DLLVM_ENABLE_NEW_PASS_MANAGER=off

LLVM_ENABLE_NEW_PASS_MANAGER is set to ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER, so
-DLLVM_ENABLE_NEW_PASS_MANAGER=off has no effect.

Change the cache variable to LLVM_ENABLE_NEW_PASS_MANAGER instead.
A user opting out the new PM needs to switch from
-DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=off to
-DLLVM_ENABLE_NEW_PASS_MANAGER=off.

Also give a warning that -DLLVM_ENABLE_NEW_PASS_MANAGER=off is deprecated.

Reviewed By: aeubanks, phosek

Differential Revision: https://reviews.llvm.org/D108775