llvmGen: Adapt to allow LLVM 15
We now must use -passes in place of -O<n> due to #21936 (closed).
Closes #21936 (closed). Closes #22220 (closed).
Edited by Ben Gamari
We now must use -passes in place of -O<n> due to #21936 (closed).
Closes #21936 (closed). Closes #22220 (closed).