Or rather, error out if it is set to something other than ON. This
removes the ability to enable the legacy pass manager by default,
but does not remove the ability to explicitly enable it through
various flags like -flegacy-pass-manager or -enable-new-pm=0.

I checked, and our test suite definitely doesn't pass with
LLVM_ENABLE_NEW_PASS_MANAGER=OFF anymore.

Differential Revision: https://reviews.llvm.org/D123126