Adding role-based capability attributes that allow you to express role management: asserting a capability is held, acquiring a capability and releasing a capability. Also includes some skeleton documentation for these new attributes.

This functionality should be considered a WIP.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201890 91177308-0d34-0410-b5e6-96231b3b80d8