[OptDiag] Split code region out of DiagnosticInfoOptimizationBase

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.

Differential Revision: https://reviews.llvm.org/D29003

llvm-svn: 293109