[clangd] Don't query index when completing inside classes

Summary:
We used to query the index when completing after class qualifiers,
e.g. 'ClassName::^'. We should not do that for the same reasons we
don't query the index for member access expressions.

Reviewers: sammccall, ioeric

Reviewed By: sammccall

Subscribers: klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46795

llvm-svn: 332226