[clangd] Code completion uses Sema for NS-level things in the current file.

Summary:
To stay fast, it avoids deserializing anything outside the current file, by
disabling the LoadExternal code completion option added in r322377, when the
index is enabled.

Reviewers: hokein

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41996

llvm-svn: 322387