[clang-tools-extra] [clangd] Avoid invalid include-fixer fuzzy-find queries (PR #212134)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> Investigated this further. `DeclarationNameInfo` is correct here: parser recognizes `operator ::align_val_t` as a conversion-function-id, and Sema constructs a `CXXConversionFunctionName` with a target type.
> 
> The incorrect step was in `UnresolvedNameRecorder`: it unconditionally called `Typo.getAsString()` and treated the diagnostic presentation string as `UnresolvedName::Name`. For this kind, that string contains the target type and can contain `std::`, which is not an outer lookup scope that IncludeFixer can split into `Name + Scopes`.
> 
> I updated the patch to reject only `CXXConversionFunctionName` before this lossy conversion. This removes the string-based `contains("::")` check. I intentionally do not reject all non-identifier names: e.g. overloaded operators may still be valid unqualified index names.

I’m not sure it’s right to always reject  `CXXConversionFunctionName`.

E.g.
`test.h`
```cpp
#pragma once
struct Something { int I = 0; };
```

`test.cpp`
```cpp
struct Wrapper {
    template <typename T>
    operator T() const { return T(); }
};

void f() {
    Wrapper W;
    auto V =  W.operator::Something(); 
}
```

test.cpp will have compilation error, which can be fixed with test.h include.
So, the question is, will we reject such a case with this patch or not?

https://github.com/llvm/llvm-project/pull/212134