[clangd] Don't assert when completing a lambda variable inside the lambda.

Summary:
This is a fairly ugly hack - we back off several features for any variable
whose type isn't deduced, to avoid computing/caching linkage.
Better suggestions welcome.

Fixes https://github.com/clangd/clangd/issues/274

Reviewers: kadircet, kbobyrev

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73960