[clangd] Fix an assertion failure in background index.

Summary:
When indexing a file which contains an uncompilable error, we will
trigger an assertion failure -- the IndexFileIn data is not set, but we
access them in the backgound index.

Reviewers: kadircet

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

Differential Revision: https://reviews.llvm.org/D55650

llvm-svn: 349144