[clangd] Fix a link failure in unittests

Summary: D46524 (rL332378) introduced a link failure when built with
`-DSHARED_LIB=ON`, which this patch fixes.

Reviewers: ioeric

Subscribers: klimek, mgorny, ilya-biryukov, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46906

llvm-svn: 332438