[lld] [lld][GNU Properties] Refactor storage of PAuth ABI core info (PR #141920)

[Peter Smith via llvm-commits]

================
@@ -961,7 +961,11 @@ static void parseGnuPropertyNote(Ctx &ctx, ELFFileBase &f,
                        "is invalid: expected 16 bytes, but got "
                     << size);
       }
-      f.aarch64PauthAbiCoreInfo = desc;
+      f.aarch64PauthAbiCoreInfo.emplace();
----------------
smithp35 wrote:

I don't think you need `emplace();` here.

Could 
```
f.aarch64PauthAbiCoreInfo = {  support::endian::read64<ELFT::Endianness>(&desc[0]), support::endian::read64<ELFT::Endianness>(&desc[8]) };
```
work.

Personally I think it is worth making aarch64PauthAbiCoreInfo a class with a constructor as there is an invariant that both of these fields need to be set. A constructor would enforce that.

Could also add a bool valid() const that could replace the calls:
```
(ctx.aarch64PauthAbiCoreInfo->aarch64PauthAbiPlatform != 0 ||
       ctx.aarch64PauthAbiCoreInfo->aarch64PauthAbiVersion != 0)
```

https://github.com/llvm/llvm-project/pull/141920