source-position doesn't make any sense for types:

source-position doesn't make any sense for types:
- atomic, pointer, function types are used throughout the program and always
  defined "on-the-fly" it's not possible to assign a correct source_position
- for struct, union and enum types we have to create a declaration_t anyway
  so they can get referenced by their name. In this case we can and should
  use the source_position of the declaration

[r23203]