[clangd] Add a #include completion test that triggers an assertion.

Summary: Test for https://reviews.llvm.org/D52774.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D52775

llvm-svn: 343593