ARCMigrate: simplify diagnostic handling

Recent enhancements in the diagnostics engine mean that
TransformActions::report() no longer needs to duplicate this suppression logic.

That's great because the old code was flawed and would have attached notes to
the wrong primary diagnostic in non-trivial use.

With these changes it becomes safe to use reportNote() freely in the migration
tool.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212191 91177308-0d34-0410-b5e6-96231b3b80d8