[clangd] Avoid a code completion crash

This is a workaround (adding a newline to the eof) in clangd to avoid the code
completion crash, see https://github.com/clangd/clangd/issues/332.

In principle, this is a clang bug, we should fix it in clang, but it is not
trivial.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D117456