[clang-tools-extra] [clangd] Add config option to allow detection of unused system headers (PR #87208)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

@vvd170501 I need to specify a commit message when merging. Github offers the PR description by default, but it seems slightly out of date (and some of it seems unnecessary).

I would like to revise it as follows, could you look it over please:

```
[clangd] Add config option to allow detection of unused angled includes (#87208)

This PR adds a new `AnalyzeAngledIncludes` option to `Includes` section
of clangd config. This option enables include-cleaner checks for all includes
that use the `<>` syntax, not just standard library includes.
```

https://github.com/llvm/llvm-project/pull/87208