[clang][Parse] Add parsing support for C++ attributes on using-declarations

This is a re-application of dc672999 which was reverted in f63adf5b because
it broke the build. The issue should now be fixed.

Attribution note: The original author of this patch is Erik Pilkington.
I'm only trying to land it after rebasing.

Differential Revision: https://reviews.llvm.org/D91630