[clangd] Add TemplateArgumentList into Symbol

Summary:
Part of re-landing rC356541 with D59599. Changes the way we store
template arguments, previous patch was storing them inside Name field of Symbol.
Which was violating the assumption:
```Symbol::Scope+Symbol::Name == clang::clangd::printQualifiedName```
which was made in multiple places inside codebase. This patch instead moves
those arguments into their own field. Currently the field is meant to be
human-readable, can be made structured if need be.

Reviewers: ioeric, ilya-biryukov, gribozavr

Subscribers: MaskRay, jkorous, arphaman, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59640

llvm-svn: 358273