cd7f7cf5cc
Reapplying now that buildbot has picked up the new configuration that does not use -treat-scalable-fixed-error-as-warning. ----- This removes the `LLVM_ENABLE_STRICT_FIXED_SIZE_VECTORS` cmake option and the `-treat-scalable-fixed-error-as-warning` opt flag. We stopped treating these as warnings by default a long time ago (62f09d788f9fc540db12f3cfa2f98760071fca96), so I don't think it makes sense to retain these options at this point. Accessing a scalable TypeSize as fixed should always result in an error.
See docs/CMake.html for instructions on how to build LLVM with CMake.