[llvm] LoongArch: Improve detection of valid TripleABI (PR #147952)

[via llvm-commits]

zhaoqi5 wrote:

> @zhaoqi5 okay, but how would I check that a warning is _not_ generated? Can you please propose a change?

I think we can delete the check directly. But it is better to find a case that still appears this warning.

By the way, I accidentally found that your changes seem to have unwanted influence to `lto`. Shows below:
```
$ clang --target=loongarch64-unknown-linux-gnu -flto=auto hello.c
warning: the triple-implied ABI is invalid, ignoring and using feature-implied ABI
warning: the triple-implied ABI is invalid, ignoring and using feature-implied ABI
```

But no warnings without your commit. I think this should be dealt with.

https://github.com/llvm/llvm-project/pull/147952