[clangd] Add a tweak for adding "using" statement.

Summary:
This triggers on types and function calls with namespace qualifiers. The
action is to remove the qualifier and instead add a "using" statement at
appropriate place.

It is not always clear where to add the "using" line. Right now we find
the nearest "using" line and add it there, thus keeping with local
convention. If there are no usings, we put it at the deepest relevant
namespace level.

This is an initial version only. There are several improvements that
can be made:
* Support for qualifiers that are not purely namespace (e.g.  record
types, etc).
* Removing qualifier from other instances of the same type/call.
* Smarter placement of the "using" line.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76432