[clang-tools-extra] [clangd] Fix sysroot flag handling in CommandMangler to prevent duplicates (PR #75694)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> Thank you for the review. Since I don't have permission to commit the change, could you merge this on my behalf?

Sure, done.

https://github.com/llvm/llvm-project/pull/75694