[clang] [Clang] Fix quadratic designated-initializer checking (PR #225573)

[Oliver Hunt via cfe-commits]

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ojhunt wrote:

I feel like I've seen index searches before, so there should probably be a method on ASTContext - something like `std::optional<unsigned> getFieldIndex(const RecordDecl*, const FieldDecl*)` and then we get

```cpp
if (auto SelfIndex = Context.getFieldIndex(RD, KnownField)) {
  FieldIndex = *SelfIndex;
} else { 
  // A field of another record: its cached index isn't RD's numbering ....
}
```

I'd kind of prefer both branches abstracted behind getFieldIndex, but I'm unsure if the direct subobject field is useful (or even expected) in other cases? If we think that's reasonable/safe then essentially this entire if/else would end on in the getFieldIndex method.


https://github.com/llvm/llvm-project/pull/225573