[clangd] Add support for the `defaultLibrary` semantic token modifier

This allows us to differentiate symbols from the system (e.g. system
includes or sysroot) differently than symbols defined in the user's
project, which can be used by editors to display them differently.

This is currently based on `FileCharacteristic`, but we can
consider alternatives such as `Sysroot` and file paths in the future.

Differential Revision: https://reviews.llvm.org/D101554