Laying the basic groundwork for table generating the diagnostics for attribute subjects. This makes some modifications to the way subjects are listed in Attr.td, and updates the attr emitter to handle the new constructs.

I have disabled some attribute subject lines on purpose in Attr.td;
this part is a WIP with the goal being to restore those subjects
incrementally. By commenting them out, it leaves the original behavior
the same as before for those attributes and so those are not
functionality changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195841 91177308-0d34-0410-b5e6-96231b3b80d8