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

[Travis Downs via cfe-commits]

travisdowns wrote:

@kleinesfilmroellchen or @ADKaster it's been quiet for a while, do you think there's any hope for this one still, or is this PR abandoned? Great work to date, BTW, this would be a big win for a project I work on as well (which unconditionally uses <> for internal headers).

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