[clangd] Also cache failures while indexing

Summary:
Clangd currently doesn't cache any indexing failures, which results in
retrying those failed files even if their contents haven't changed.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63986

llvm-svn: 365120