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

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the report and patch. Could you please

* Change the description to include the explanation at https://github.com/llvm/llvm-project/issues/147665#issuecomment-3051584839 , with potential simplification, so that future contributors reading the commit message do not need to click the link?
* Add a test to a `clang/test/Driver/loongarch-*.c` file (perhaps loongarch-abi.c) ?

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