Code region is the only part of this class that is IR-specific. Code
region is moved down in the inheritance tree to a new derived class,
called DiagnosticInfoIROptimization.
All the existing remarks are derived from this new class now.
This allows the new MIR pass-remark classes to be derived from
DiagnosticInfoOptimizationBase.
Also because we keep the name DiagnosticInfoOptimizationBase, the clang
parts don't need any adjustment.