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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

What is the relationship between this patch, and clangd 17's ["missing include" warning](https://clangd.llvm.org/guides/include-cleaner#missing-include-warning)? Does the quick-fix for the "missing include" warning also respect these config options?

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