[clangd] Don't consider class template params part of constructor name.

Summary:
This is shorter and usually the extra info is noise.
There are cases where the params become type-parameter-0-0 that are hard to fix.

This affects a few features:
 - 'name' field in structured hover API (not exposed yet)
 - 'name' field in locateSymbolAt (not exposed in LSP)
 - 'document/symbol' - the symbol is hierarchically nested in the class
   template, or written as foo<t>::foo when defined out-of-line.

Added a test case for hover from https://github.com/clangd/clangd/issues/76.
This patch fixes one field, but no fewer than four others are wrong!
I'll fix them...

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70308