[llvm] [LoongArch] Add codegen support for ILP32D calling convention (PR #141539)

[Lu Weining via llvm-commits]

https://github.com/SixWeining approved this pull request.

I'll approve this although the 32bit ABI has not been released because llvm still issues  warning like:
```
warning: 'ilp32d' has not been standardized
```
and this warning will be removed once the ABI is standardized and released.

BTW, does the clang part needs modification? E.g. `clang/lib/CodeGen/Targets/LoongArch.cpp`. But at least we should add some new tests like the existing `abi-lp64d.c`.

https://github.com/llvm/llvm-project/pull/141539