[clang][Index] Visit UsingDecls and generate USRs for them

Summary:
Add indexing of UsingDecl itself.
Also enable generation of USRs for UsingDecls, using the qualified name of the
decl.

Reviewers: ilya-biryukov, akyrtzi

Subscribers: arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58340

llvm-svn: 354878