[clangd] Fix a reference invalidation

Summary: Fix for the breakage in http://green.lab.llvm.org/green/job/clang-stage1-configure-RA/52811/consoleFull#-42777206a1ca8a51-895e-46c6-af87-ce24fa4cd561

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

Differential Revision: https://reviews.llvm.org/D56656

llvm-svn: 351052