[clang] [Sparc][clang] make `_Complex` ABI GCC-compatible (PR #212340)

[Folkert de Vries via cfe-commits]

https://github.com/folkertdev commented:

> I'm not sure we need `-fclang-abi-compat` flag, but I'm not an interested party.

It's just a courtesy, and quite cheap to support. I'm happy to remove it though if it's not required. (it was the recommended strategy for a similar change in the powerpc backend, see https://github.com/llvm/llvm-project/pull/77732#issuecomment-4938515656)

> Also, are `_Complex types` passed in memory the same way? (when we'are out of argument registers)

the same way as what?


https://github.com/llvm/llvm-project/pull/212340