[clangd] IncludeCleaner: Don't warn on system headers

[clangd] IncludeCleaner: Don't warn on system headers

This is a temporary hack to disable diagnostics for system headers. As of right
now, IncludeCleaner does not handle the Standard Library correctly and will
report most system headers as unused because very few symbols are defined in
top-level system headers. This will eventually be fixed, but for now we are
aiming for the most conservative approach with as little false-positive
warnings as possible. After the initial prototype and core functionality is
polished, I will turn back to handling the Standard Library as it requires
custom logic.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D112571