[clangd] Add RemoveUsingNamespace tweak.

Summary:
Removes the 'using namespace' under the cursor and qualifies all accesses in the current file.
E.g.:
  using namespace std;
  vector<int> foo(std::map<int, int>);
Would become:
  std::vector<int> foo(std::map<int, int>);

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68562

llvm-svn: 374982