[clangd] Implementation of workspace/symbol request

Summary:
This is a basic implementation of the "workspace/symbol" request which is
used to find symbols by a string query. Since this is similar to code completion
in terms of result, this implementation reuses the "fuzzyFind" in order to get
matches. For now, the scoring algorithm is the same as code completion and
improvements could be done in the future.

The index model doesn't contain quite enough symbols for this to cover
common symbols like methods, enum class enumerators, functions in unamed
namespaces, etc. The index model will be augmented separately to achieve this.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: jkorous, hokein, simark, sammccall, klimek, mgorny, ilya-biryukov, mgrang, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D44882

llvm-svn: 330637