[clangd] More precise representation of symbol names/labels in the index.

Summary:
Previously, the strings matched LSP completion pretty closely.
The completion label was a single string, for instance. This made
implementing completion itself easy but makes it hard to use the names
in other way, e.g. pretty-printed name in synthesized
documentation/hover.

It also limits our introspection into completion items, which can only
be as precise as the indexed symbols. This change is a prerequisite to
improvements to overload bundling which need to inspect e.g. signature
structure.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D48475

llvm-svn: 335360