[clang-tools-extra] [clangd] Allow specifying what headers are always included via "" or <> (PR #67749)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 commented:

I took a pass through the implementation. Generally looks good.

Would be nice to have a test that exercises the include insertion process in a more end-to-end way. I think something in `CodeCompleteTests.cpp` would work, using `WithContextValue` to set config values ([example](https://searchfox.org/llvm/rev/365aa1574a1b4a3cdee6648227d095d00536ffde/clang-tools-extra/clangd/unittests/InlayHintTests.cpp#77-84,100)), and using the [`insertIncludeText`](https://searchfox.org/llvm/rev/365aa1574a1b4a3cdee6648227d095d00536ffde/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp#83) matcher to make assertions about the spelling of inserted includes.

https://github.com/llvm/llvm-project/pull/67749