[clangd] Respect `--sysroot` argument if it is set

Summary:
- Since `--sysroot` is a superset of the `-isysroot` argument, we
  shouldn't add the `-isysroot` if we detect a `--sysroot` flag.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72415