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

[Sergei Barannikov via cfe-commits]

s-barannikov 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.


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