[clangd] Log dropped diagnostics.

Summary:
clangd drops diagnostics coming outside the main file, but it is still
useful to see that something went wrong in the logs.

Reviewers: hokein, ioeric, sammccall

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42803

llvm-svn: 323992