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

[Bevin Hansson via cfe-commits]

bevin-hansson 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.

Maybe this is related to how the standard library headers are indexed somehow...

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