[clangd] Add access specifier information to hover contents

[clangd] Add access specifier information to hover contents

Summary:
For https://github.com/clangd/clangd/issues/382

This commit adds access specifier information to the hover
contents. For example, the hover information of a class field or
member function will now indicate if the field or member is private,
public, or protected. This can be particularly useful when a developer
is in the implementation file and wants to know if a particular member
definition is public or private.

Reviewers: kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80472