[clangd] Drop injected class name when class scope is not explicitly specified.

Summary: E.g. allow injected "A::A" in `using A::A^` but not in "A^".

Reviewers: kadircet

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

Differential Revision: https://reviews.llvm.org/D55065

llvm-svn: 347982