[clangd] Improve type printing in hover

Summary:
Do not include tag keywords when printing types for symbol names, as it
will come from SymbolKind.
Also suppress them while printing definitions to prevent them occuring in
template arguments.
Make use of `getAsString`, instead of `print` in all places to have a consistent
style across the file.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72450