Rename attribute related instance variables to better express intent

`@attributes` was actually used for `_before_type_cast` and friends,
while `@attributes_cache` is the type cast version (and caching is the
wrong word there, but I'm working on removing the conditionals around
that). I opted for `@raw_attributes`, because `_before_type_cast` is
also semantically misleading. The values in said hash are in the state
given by the form builder or database, so raw seemed to be a good word.
Merge pull request #15427 from sgrif/sg-rename-attribute-ivars

Rename attribute related instance variables to better express intent