[ffigen] Fix record use mapping

[dcharkes]

Generate the mapping after FFIgen has run, so that we can properly rely on the renamers.

Fixes the code assets sample. (Note this sample isn't tested with dart on the CI because record use is still experimental and that would break on changes.)

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
code_assets	None	1.0.0	1.0.0	1.0.0	✔️

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[liamappelbe]

I don't like having state that's set during codegen. I think the correct approach is to make funcVarName a proper Symbol, rather than an ad-hoc private name. Then resolvedRecordUseKey can be deleted and recordUseMapping can determine the key itself. This would also let you revert the changes in writer.dart.

Aside from code cleanliness, this is also a better approach because Scope.addPrivate is only supposed to be used for internal names. But the record use map is now exposing that symbol, so it should use the Symbol infra.