When preprocessing was integrated to llvm-rc in 2021, this was a
new requirement (previously one could execute llvm-rc without a
suitable preprocessing tool to be available).
As a transitional helper, llvm-rc fell back on skipping preprocessing
if no suitable tool was found (with a warning printed), but users
could pass an llvm-rc specific option to silence the warning, if they
explicitly want to run the tool without preprocessing.
Now 2 years later, remove the transitional helper - error out if
preprocessing failed. The option for disabling preprocessing remains.