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

[Lei Huang via cfe-commits]

lei137 wrote:

> I think the issue could be handled a different (more generic) way, by pulling `useFirstFieldIfTransparentUnion` to the caller and taking transparent unions into account when emitting LLVM IR for the formal / actual parameters somewhere in `CGCall.cpp`, so that ABIInfo implementations don't need to care about transparent unions at all.

I am not familiar with the calling convention and ABI to know if this is how it should be done.  It sounds like a reasonable improvement in how we can implement this.  However I feel like it is out of scope for what I am trying to do right now.  Would this be better as a separate PR to change how this is handled?  This PR would fix the current codegen bug that is seen.

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