[clang] [llvm] [PowerPC] Fix codegen for transparent_union function params (PR #101738)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

With `--target=powerpc-unknown-linux-gnu`, the union type is still being used with `signext` (causing the IR verifier to be unhappy) for the following:
```c
typedef union __attribute__((__transparent_union__)) U {
  signed _BitInt(17) x;
} U;

__attribute__((__weak__)) long f(U u) { return u.x; }
```

I think this is primarily an artifact of `_BitInt` not being properly handled for AIX and PPC32 Linux though.

https://github.com/llvm/llvm-project/pull/101738