[clang] Fix codegen for transparent_union function params (PR #104816)

[Amy Kwan via cfe-commits]

https://github.com/amy-kwan approved this pull request.

LGTM for using `CGT.ConvertType()` like you did previously. I agree that if we were to do a larger/refactoring patch, that can be separate since this PR fixes the current codegen.

https://github.com/llvm/llvm-project/pull/104816