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

[Andrew Kaster via cfe-commits]

ADKaster wrote:

@HighCommander4 @sam-mccall do you have any capacity to review this change? It would be nice to get it into the 18 release if the window hasn't closed.

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