[CodeComplete] Constructor overload candidates report as vector(int) instead of vector<string>(int)

Summary:
This is shorter, shouldn't be confusing (is consistent with how they're declared),
and avoids messy cases that are printed as myclass<type-param-0-0>(int) in the
case of partial specialization.

Fixes part of https://github.com/clangd/clangd/issues/76

Reviewers: hokein, lh123

Subscribers: ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70307