Strip user metadata before codegen passes

Any tags that are not used in code generation should likely be stripped before entering into code generation. Otherwise, we'll get cache misses (and run code generation redundantly) for no good reason.

Likely, this presumptive `strip_user_tags` should be offered as a "transformation", too. I'm not clear on whether we should leave this to the user or do it automatically, but I'm biased in favor of *not* doing it automatically.

cc @kaushikcfd @nchristensen

The issue names no files, tests, or entry points. First locate where user metadata enters the code-generation pipeline and how transformations are represented; clarify whether stripping is automatic or an opt-in transformation, then define coverage for the selected behavior.

Written by the indexing model from the issue text.