Merge changes from topic "handle-odr"

* changes:
  Update tests after type-ids were introduced to refer to types.
  Fix abi diffing to incporporate changes in type graphs.
  Make header-abi-linker merge type graphs.
  Add support to TextFormatToIRReader to merge type graphs.
  Move DiffWrapperBase into a seperate utility class.
  Use type-ids to refer to types instead of their names.