[clang] [Sparc]: don't pass packed structs or unions via FPRs (PR #214894)

[Daniel Cederman via cfe-commits]

doac wrote:

I compared the output with GCC and this looks correct to me.

Codex noted that it misses the case with packed C++ base classes.
```
struct packed_base {
    float f;
} __attribute__((packed));

struct derived_with_packed_base : packed_base {
    float g;
};
```
Would be good to cover this, or leave a note about it somewhere. But otherwise this LGTM.

https://github.com/llvm/llvm-project/pull/214894