[clangd] Filter out private proto symbols in SymbolCollector.

Summary:
This uses heuristics to identify private proto symbols. For example,
top-level symbols whose name contains "_" are considered private. These symbols
are not expected to be used by users.

Reviewers: ilya-biryukov, malaperle

Reviewed By: ilya-biryukov

Subscribers: sammccall, klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46751

llvm-svn: 332456