[clangd] Expose 'shouldCollectSymbol' helper from SymbolCollector.

Summary: This allows tools to examine symbols that would be collected in a symbol index. For example, a tool that measures index-based completion quality would be interested in references to symbols that are collected in the index.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D48418

llvm-svn: 335218