[clang] [clang][LoongArch] Match GCC ABI handling of integer complex types (PR #215222)

[via cfe-commits]

heiher wrote:

> LGTM, thanks. Should we also update the ABI documentation to explicitly describe the calling convention for GNU integer complex types? @SixWeining

Thanks! I followed this rule:

https://github.com/loongson/la-abi-specs/blob/44ef5f63e1755e1a165915ea3b2d98d35f68eed0/lapcs.adoc?plain=1#L565-L569

```markdown
*Structures without floating-point members*

* 0 < w~arg~ ≤ GRLEN

** The argument is passed in a GAR if there is one available with the members
laid out as if they were stored in memory. Otherwise, it is passed on the
stack.
```

https://github.com/llvm/llvm-project/pull/215222