[clangd] Don't treat top-level decls as "local" if they are from the preamble.

Summary:
These get passed to HandleTopLevelDecl() if they happen to have been
deserialized for any reason. We don't want to treat them as part of the
main file.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54303

llvm-svn: 346503