[clang-tools-extra] [clangd] Augment code completion results with documentation from the index. (PR #120099)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> I've come across an issue with this, again. This seems to work for headers I've written myself (or at least ones in my own project), but for e.g. standard library include types, it doesn't work. Something like
> 
> ```
>   std::string a;
>   a.^
> ```
> 
> still contains no documentation in the autocompletion results even though there are doccomments in the header in question.

Seems to be template-related.

test.hpp:
```c++
template <typename T>
struct templated {
  // comment
  void foo();
};
struct nontemplated {
  // comment
  void foo();
};
```

test.cpp:
```c++
#include "main.hpp"
void foo() {
  nontemplated n;
  n.f^   // comment is shown
  templated<int> t;
  t.f^  // comment is not shown
}
```

https://github.com/llvm/llvm-project/pull/120099